If you have a good collection or just one individual Barbie the best place to sell them would be on ebay. Ebay is probably where you will get the best price for your Barbie. People looking to complete or start a collection would look there first. You can start your bid at whatever amount you want or you can list for a buy now price. Either way you list it ebay will be the best place to sell them.
You can have a garage sale and sell some of your toys you don't want, and then you could get good money to get other toys :D (i'm guessing you're a little girl lol) But make sure your dolls are in good shape and condition, otherwise people might not even bother looking at them.