S.H.
I noticed ebay will get you the most money. You can set a minimum and then people bid. If you are not an ebay person (it is an effort, yet it pays off) then consignment is much easier.
Consignment is pretty low. A $100 dollar dress at Bebe could list for $40 and you (the consigner) will get a percent. I think 40% cash or 60% store credit is common. So you will $16 in this scenario. edit: There is no guarantee what it will sell for, so $40 is a guess. If it sells for $10 you will get $4. You can ask what they plan to sell it for at the store.