In D&D, much as in real life, how well you're doing is ultimately contingent on how much money you have.
So much for a fantasy game, huh?
No joke, though, a character's worth will generally comes from two places: magic and magic items. (The fact that one of these is not inherently available to all characters is a small reminder on this system's balance.) Monks, oft maligned as the bottom of the barrel for core classes, need a boatload of practically standardised magic items just to bring their class features up to a passably-functional level. Druids, meanwhile, do go gently into that good night as naked as a jaybird - and in the form of a jaybird, too, at their option.
So if you want to get rich quick, what are your options? If you look at the Craft, Perform, and Profession skills, even astronomical DCs still result in paltry income. Fortunately, as is the case with any sufficiently large system that has enough different minds behind it, there are exploits in place that can be capitalized on by the players. It's probably safe to assume that none of these were intended by the game designers, but as is always the case, too bad ha ha ha.
Presented here, then, is a handy list of Ten 100% rules-legal, 100% obscene methods for amassing in-game wealth. You may not be rich, but you can at least pretend to be for one night a week.
1. The "Don't Dispel a Gift Horse In The Mouth" Method
Mount is a level one spell that lets you summon a light horse for 2 hours per level. The going price for a light horse is 75 gp. It even comes with a bit & bridle (2 gp) and a riding saddle (10 gp.) Being able to produce 87 gp of salable goods at level 1 is pretty amazing, and two hours is plenty of time to make yourself scarce.
Your DM is probably going to want some Bluff checks when you try to sell it off, and you might want to make use of disguise self before the transaction if you're going to do it in a town you ever want to be seen in again.
2. The "Magic Beans" Method
Magic aura is a level one spell that can make a mundane item register as magical. If you can make a good Bluff check to a spellcaster (of sufficiently low level that he won't see through your ruse, but high-enough level he'll have cash to burn) you can sell it as magical, too. Especially nice for Bards, who are charisma-based and also get glibness. Again, just make sure you're far away when the duration of magic aura expires.
3. The "Negative Labour Cost" Method
An iron pot weighs ten pounds and costs five silver pieces. However, under "trade goods," one pound of iron costs one silver piece. Don't ask; these are the same tables that claim an empty flask weighs more than a flask filled with holy water. Melt down an iron pot into ten pounds of iron, trade them for their equivalent value, and get a 5 sp profit. Continue until infinite money.
Don't be too surprised if your DM says "obviously, no," and puts a stop to it, but it can be a fun one to pull if he's an agonizing stickler for Rules As Written.
4. The "Less Than the Sum of its Parts" Method
A 10-foot pole costs two silver pieces. A 10-foot ladder costs five copper pieces. Buy a ladder, remove the rungs, and you now have two 10-foot poles. Sell for a profit of 3.5 sp and repeat. Infinite money potential in even a moderately-sized city.
Even if it's determined that you aren't left with two functional poles, you have to admit that that pricing model makes no sense just in terms of cost of materials.
5. The "Don't Be Salty, DM" Method
Wall of salt is a level 4 spell that lets you make a (hope you're sitting down for this one) wall of salt, at the rate of one five-foot square per level and one inch of thickness per level. While perfectly respectable in terms of its combat potential, the instantaneous (and thus permanent) wall has other, more interesting applications. Salt is worth 5 gp per pound, and seeing as it's a trade good, is always 100% of its listed price in transactions. Even at minimum caster level (7th) that's 25 250 gp per casting.
6. The "One-Man Armory" Method
Cast wall of iron, which you'll need to be at least level 11 to do. Then cast fabricate, converting eleven cubic foot of the wall into masterwork daggers. The density of iron is 491 pounds per cubic foot, so 44 cubic feet will weigh 21 604 pounds. At one pound per dagger, that's 21 604 masterwork daggers, which sell for 151 gp each. Even one Wall of Iron will net you over three million gold pieces at level 11. You can hire the now-unemployed blacksmiths to Aid Another on your future fabricate checks.
If your DM starts stammering about "demand curves" or "market saturation," teleport and plane shift could help you sell to an infinite multiverse's worth of prospective buyers.
7. The "Homemade Goods" Method
The creation spells, both Minor and Major, allow you to create vast quantities of raw material from a small amount. Major creation even lets you create full-on gems, which will sell for a heck of a lot more than a light horse. Sell all you can, then get out of dodge before the duration expires.
One interesting idea is that a level 7 wizard could use minor creation to create 7 cubic feet of saffron. This site gives saffron's density to be about nine pounds per cubic foot, totaling 63 pounds, or 945 gp worth per casting. So long as it's eaten before the 7 hour duration expires, who will know the difference? Supply all the kitchens and eateries in the kingdom, and you'll never have to disappear for selling temporary goods.
8. The "Help Me With My Homework" Method
Planar binding will let you keep an Efreet or Noble Djinn in your basement and shake him down for infinite wishes. Wish lets you create items of up to 25 000 gp in value, so a free 75 000 gp per day is nothing to sneeze at.
Even if wish is off the table, a regular Djinn can still cast major creation every day... and any vegetable material created is permanent. Suddenly you can start shipping saffron to other continents! Even better, Black Lotus Extract is worth 4500 gp per dose. Assuming a vial of poison is about an ounce and one cubic foot = 957 fluid ounces, then 20 cubic feet of the stuff is equivalent to 19 140 vials, or a market value of 86 130 000 gp.
9. The "Free Labour" Method
Undead, being mindlessly obedient and completely untiring, are so convenient. Any form of brute labour, such as mining or quarrying, is more profitable when it's performed by workers who can function 24/7 and need no pay or even upkeep. Even if there are no mines nearby, walls of Stone/Salt/Iron that you cast can be quarried for sale.
Interesting options open up if you can get an intelligent undead cohort (or a warforged, or someone otherwise tireless.) Use multiple walls of stone to put the raw materials in place, and have your follower play a Lyre of Building for 24 hours straight. This nets the effective effort of 100 people labouring for 144 days: virtually any earthly construction project should be doable in that time. Even if you only charge half the projected project cost, being able to build a castle in a day should be pretty profitable.
10. The "Glowsticks For All" Method
Lantern Archons, in addition to being cute, can cast continual flame at will, and can be created (not just summoned) with the spell create lantern archon. Acquire a large number of mundane sticks/rods (fabricate can help, although... hey, you didn't throw away all of those ladder rungs, did you?) A created Lantern Archon is happily willing to perform non-hazardous services for an hour, so it should have no problem casting continual flame 600 times. The profit from selling hundreds of Everburning Torches will easily fund that casting of restoration to recoup the sacrifice cost of create lantern archon at the end of the day.
Of course, scams have the potential to be found out, and even 'the perfect crime' could have the DM arbitrarily putting a halt to it (and not without reason, either.) Exploiting valuable trade goods will just lead to market saturation, so if your DM happens to stumble upon a Wall Street Magazine article in his basement toilet, he will recognize the law of supply and demand which states that while the demand for said items is the same, the supply just skyrocketed, making them virtually worthless. If you want to make a fortune in D&D, the best way is to go adventuring; most characters quickly stumble across vast amounts of wealth with little-to-no difficulty (other than the frequent need to part said wealth from its current owners).
Adventuring is the ultimate get-rich-quick scheme.