Who are you to get to decide what prices should be? It's up to the individual seller to place a value on what they're selling.
Even with your system I could get around it easily if I wanted to crash a market simply by playing leapfrog between characters. Character A posts item for 100 gil. Switch to character B and post same item for 90gil. Switch back to character A to post for 81gil and so on until I get the item to the price I want to sell it for.
Some players sell low simply because they want the item out of their inventory, not because they're trying to crash the market. It's the people who panic over being undercut that cause the problem when leaving your item at your preferred price will usually lead to it being sold at that price. I go through that all the time with the chocobo feed and color fruits I sell to raise gil for my alt's FC. Sometimes someone comes along and undercuts me, everyone else panics and does the same but I leave mine alone and a day or two later they've sold at my preferred price. If they don't start selling again within a few days, then I'll check the current prices. It could be that the available supply is too high and I'm better off pulling mine off the MB for a week before listing again. Certainly I stop growing more and switch to other items while I wait.
If you think the listed price is too low, buy up that item and resell it for the price you think it's worth. If you keep getting undercut, then your price is likely too high compared to the demand for that item.
