You can purchase domains via various domain auction sites, these websites make it easier to buy and sell domain names.
A drop registrar is a domain name registrar who registers expiring domain names, to sell at auction, as soon as the registration expires. To participate in a drop registrar auction you need to register your interest prior to the beginning of the auction.
A domain auction site allows a customer to purchase a previously registered domain, which suits their needs, from an owner who wishes to sell their domain name. A domain auction site allows sellers to list multiple domains, easily and conveniently, on the one website.