If you're selling a domain, almost all domain registrars offer free transfer across their services.
To sell the domain, you can either post here in the Digital Goods section or you can Auction it off in the
Auction section. Once you have a buyer or winner, you can either decide which of you wants to transfer the domain/funds first, or you can use an escrow.
Post the domain and things such as a price you would like, the expiration date of the domain and such to gauge interest and then you can start selling.