So basically a 2 way system on the marketboard where you can instantly buy (as it is now) and instantly sell to another player at a price/quantity that they dictate?

I wouldn't be against it, players can choose whether or not to sell to these players, they get an instant sale, the buyer gets their item instantly. If you don't like the buy offer, you can sell at a higher price and a quantity you like.

Would this not theoretically just do the opposite to undercutting though? People posting buy offers by 1gil higher so they buy everything first? You would also maybe need a cap on the number of postings active per account, because what would stop me posting 1000 buy offers and not giving anyone a chance to have their name on the buyers market? Being wary of advertising other games, but I suppose others have done so in this thread so maybe not as voodoo as I'd have thought, but a game called Shop Heroes is a very good example of a buyers and sellers market but I guess that game is literally 95% revolved around it, so it had to be suitable..