Sellers must pay a one-time fee that covers the overhead of double-checking to make sure a seller is a legitimate person with a legitimate shipping address. After that small fee, selling is free.
Paid accounts receive additional benefits, but it is not necessary to have a paid account to sell. When an account is in paid status, it has all of the benifits of paid accounts, such as unlimited auctions and a storefront. (Click here for a full list of the benefits of a paid account)
Currently, Paid accounts are $5/month, or $30/year.