:/
This is why you have a referee.
Player: I read off the scroll of Gate, bring in an Efreet. My first two wishes are for two more Gate scrolls, and my third wish is for a +5 Vorpal Scythe.
DM: I get what you're doing. Do you guys still want to play this game? Because I have things I can do on a Saturday night.
Player: Well can I get away with it just once?
DM: Uh, let's see. Trade a 4,000 GP scroll for three 200,000 GP scythes. All signs point to no.
Honestly, does anyone ever actually do these stupid things or do people just perpetually wank about them on the WotC forums? I have come across several dozen player scams, often me doing the scamming, over the years. We just end up making a gentleman's agreement not to do it.
One very simple, perhaps the most basic example. This is from 2E where Fire Trap doesn't have an expensive material component:
Fire Trap is cast on a container or enclosure. It goes off if the enclosure is opened. But clearly it should go off if the enclosure is bashed open, also. You cast it on a chest, and if someone opens the chest or breaks it, the Fire Trap goes off.
So the scam is to cast Fire Trap on a small clay jar. You put more clay jars, with Fire Trap, in the bag. It has to be heavy enough to throw. Each jar is sealed with pitch to be airtight. Now in each jar you stick a tight bundle of tiny paper envelopes, glued shut, and tied with string. Each of these dozen envelopes has a Fire Trap cast upon it.
Now you throw the bag at something. Each of the clay pots in the bag needs to save vs. crushing blow. If one of them fails, the flames erupting from it will burn the pitch off the others, opening them and burning the paper bundles, which sets off the Fire Traps. If you break two jars, it goes off instantly, but if you break just one then the DM might rule that the fire has to take a round to burn through the pitch and ignite the envelopes of the other jars.
1d4 damage +2 per caster level. This can be cast by a 3rd level Cleric. Ten jars per bag, 12 envelopes per jar. That's 120d4+720, save for half, in a 5' diameter explosion.
Sure there are risks associated with carrying around these things. Don't get hit by fire! But it can really screw with the game. So our agreement was that it was cool so long as you didn't have more than one Fire Trap per throw. So Fire Trapping a big flask of oil was perfectly fine.
And we were cool with that.