But you see that is the problem, the items in your bags, the enchants, the gems the foods that transmog gear (if your on Cata or MoP), or the extra gear you have, the gold your character carries, those mounts and pets collected etc etc... those things all have their cost, they cant ignore that when they calculate the price.
But players selling their characters can do something about it, you can delete or sell items from bags, transfer gold to another character, that will bring down your character price, you cant get rid of mounts and pets but that only makes your character more desirable. Didn't check if they implemented it, but list of mounts and pets would be a great idea.
When you are buying items from marketplace you don't get those items enchanted and gemed, when you buy lvl 80, 85 or 90, you get 0 achievements 0 mounts etc... here you are not just buying a blank character.