How do different sites sell the same domain name?

[.]

I know that I can buy the same domain name from many sites. How do they all have rights to the same one?

Also, how do individuals that buy and sell domain names fit into this? Are they just sellers for the big sites?

[Chris]

They all check the same master DNS lookup tables (WHOIS/ICANN). If nobody has a claim on that domain, they can market it.

Individuals that typically resell domains purchase the potentially popular domain name -- it is registered to them and they 'own' it. Thus, they can then resell the domain name for whatever sum they choose.

[PokerPlayer]

They all are checking to see if its available from the same database.