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? Or do you want to buy one second hand?
I do not recommend obtaining a used access key. First, know that trading in Guild Wars accounts is against the NCSoft EULA, so if you should get one second-hand, and if ArenaNet should find out, then you run the risk of that account being closed immediately. Also, you have no way of knowing that illegal activities that account has already been used for... the account could be closed any day because of things done on it in the past.
And nowadays you can already buy a brand new license key for one campaign for around $10, for the Guild Wars Trilogy (=the 3 campaigns) for around $30, and for the Guild Wars Complete Collection (=the 3 campaigns+the Eye of the North expansion) for just slightly more. That's a really good deal, especially considering that the game doesn't have montly fees. So why trouble with a used account at all?