I would say that for me, it would be most important to have *no* fees. Sites like Ebay could make plenty of money in other ways.
I don't mind reasonable fees. If they are acting as middleman for me
to sell my things, I don't think they should have to make money other
ways or they'd be in another business.
Another thing that I didn't see mentioned yet, is that I think there
should be a cap on what is allowed to be charged for shipping. It may
be impossible to enforce, but they could probably have some kind of
chart that lists weights and shipping costs for 1st class... make sellers
have a weight listed and then make sure the shipping cost is reasonable
and not something like $10 for a ring that weighs .5 lb. or something.