If you mean you have a domain and an interested buyer, you can just do a deal with the buyer, you don't need to "use" any intermediary.
If you have a domain for sale and you want to find a buyer for it, you can list it on Ebay, which has a category just for domains.
If you have a domain that you want to use for yourself and you don't want to sell it, but someone is trying to buy it from you anyway, BE CAREFUL. There are a lot of traps in this situation and you can end up losing your domain with no compensation if you make an incorrect move. Better talk to a specialist.