I could see it being not too unbalanced. Give job exp (not base!) for the transaction, proportional to the zeny gained. Make it a small fraction, like 0.1% of the value. Selling something for 10m would be less than two leaf cats. Given that we have a tax of 2% or thereabouts, even getting 1% of the money as job exp would be monetarily inefficient for powergrinding beyond normal means. Mammonite is better exp returns.
Kroggles ensures the living stay living, and the dead stay dead.
Clobberella beats you up.
Teela Brown has a birdie!
Flosshilde communes with the very souls of the damned.
Walsung is actually pretty cool. For a guy made of metal.
In which we wonder, why do we use so much time to change the hardcoding of how vending works (note: taxes is pre-made for us, we just fill in the % number) just to get something that's inefficient? There's quite a bit on our to-do list...
i personally neither support nor am against giving exp for vending. i was just pointing out that the exp gain from doing such would be rather insignificant, whether base or job exp. granted you are getting something for "free", but just think of how much easier, and faster it would be to just go out and grind for the levels and exp rather than sell all your crap that will either not be sold because someone is giving the better deal or you're the one giving the better deal meaning less money and less exp gain or as a third alternate situation no one wants what you're selling so you're gaining no exp anyway.
even if you are selling the items people want (tool shop items/restorative items/etc) and selling them for a bit more than DC price you won't make a fortune, nor any real exp even when they are sold in mass quantity.
would it be free exp with no real effort put in? yes it would which does go against the rules of the community.
would it be worthwhile to get this free exp? no, not at all. it would take far longer than it would going out fighting enemies and gaining exp and money that way.