Domain name marketplaces are websites where you can purchase domains that are already registered and owned by someone else. In most cases the process works like any other e-commerce website, if a price is given. Or in cases where there is no price, there is a way to contact the owner and make an offer or request the price. This is the best way to acquire domains you want but are not available to register. The following are lists of mainstream and boutique (or specialty) domain marketplaces.
Mainstream Domain Marketplaces
Afternic
GoDaddy
Sedo