2nd Go Round is my favorite place to sell used baby and kids items. They offer the best prices and are generally in the market for the things I want to sell. Their store is full of treasures and it's a fun place to spend an hour or two browsing for deals.
If you have strollers, furniture or toys to sell, this is your best bet. If you're looking to buy gently used baby items at great prices, take a look here.