This is a horrible suggestion.
If you want to negotiate prices/quantities then stand near the summoning bell and shout for someone to negotiate with you.
But that would be inconvenient and tedious........EXACTLY.
The MB is a convenience for the Buyer and a risk for the Seller. The way it operates reflects that. If I want to unload a stack of something quickly, I put a full stack up cheap. I do this because I want to unload the WHOLE stack, not sit around while people pick it off 1 by 1 in 99 transactions. If I want to sell in "perfect sized stacks" i.e. 4 logs (exactly the requirement to make lumber) then I put up logs in lots of 4 each and jack up the price for the CONVENIENCE of the buyer being able to get exact quantities of what they need and the RISK that a) Someone may buy a larger stack at cheaper anyways and b) the fact that I can sell less items simultaneously thus reducing my potential income in between checking retainers.
In example: (Numbers random and arbitrary) Say Market on Pine Logs is 600gil each at the moment. If I want to unload a full stack of Pine Logs quickly, I will put them up as a full stack for well below market, say 450 each to incentive-ize either a) Someone who only wants 40 - 50 to buy the full stack and do want they want with the extras or b) Someone who plays the MB to buy the stack and re-sell at profit. However, if I want to maximize my profits, I will find what the lowest cost of a "perfect" stack (4) is and sell at or below that price, so maybe in this case the lowest price of someone selling a stack of 4 is 1,000gil each; I'd sell my stack of 99 in many sets of 4 for 998 each.
In other words, if you let people start to pick and choose quantities to buy from a posting, I and everybody else involved in the Marketboard would start to default to premium prices and EVERYTHING would skyrocket in price, which hurts you even more because now you are paying a premium for everything with no opportunity to buy in bulk cheap and turn it around for profit.