Bukkit

a way for shop signs to buy a material without checking for a quantity and after a certain amount of buys the sign makes the price then drops, and this occurs everytime the player has sold a certain amount of that item.
at the same time every 5 minutes[MineCraft Day] the price on the sign resets, and as long as no on esells to the sign for a whole minecraft day the price increases a certain percentage every minecraft day as long as no one sells to that sign.

So essentially what you want is about the same as what the current inflation/deflation system does, however you want to have this set up in a way that does not use the item stock, but instead the amount of purchases/sales made?

This sounds like an interesting idea, although it would technically be obsolete as the current inflation/deflation system works nearly the exact same...

As for the sign system, GiantShop currently does not officially provide support for signs... Sorry :(