TF2OP is a free market, so users can set any prices they wish. Having an up or down vote system would potentially harm other users trading because if someone doesn't like a trade, they could go through and down vote all of their currently open trades.

I agree with Nooge, if a user does not like a person they have dealt with they can go to every trade. Plus a lot of people seem to think if it is not the price of backpack.tf then it is a rip off however, it depends on the rarity of the item and whether a person can get enough evidence to have the price raised. They are a lot of users who don't use backpacks prices but that does not mean they are taking advantage of another user. I do see your point in some ways but unless you know someone is really sharking then there isn't much you can do about it. Have a great day/night.