If they could, then Nintendo should do what prunescape does, when you put items up on the grand exchange in runescape, you can only ask for how much it's worth, they should do that with the GTS.
Say if you've got a starter pokemon(eg, Chimchar)then you can only ask for a pokemon of equal or grater value, say a Mudkip, or somthing, that would make it a bit better, but i'm not sure how it would work