Guild Wars access keys cost money (though GW doesn't have a monthly subscription fee, so it's just a one time investment). Are you suggesting that people just give you an access key that they themselves had to pay money for?
Also, I do not recommend buying a used access key. First, reselling of accounts is against the NCSoft EULA (so if they ever find out, you run the risk of your account being terminated immediately), second, you have no way of knowing what 'illegal activities' the account has already been used for (again, the account could be terminated any moment because of previous actions done on it).
Note that you can already buy Nightfall new & unused for around $10 (or you can buy the Guild Wars Trilogy, which includes Nightfall, Factions and Prophecies, for around $30).