This looks extremely fishy: They are not selling you a game, but an "account" that has the game. So I see several scenarios:
1) These guys hack into other people's accounts and passwords and resell them
2) They sell you an account that you might be sharing (without knowing it) with other people.
2) Is the most likely scenario: they sell the credentials of one account to several people. Then, they might have a way to find how often you use the game. Once they see you are not playing the game too much (e.g. your last login with their account was a week ago or something), they sell the account to somebody else. If that somebody else plays at different times of the day, etc... you might not notice, until the time you try to login at the same time as them. This is why they have the "other" option, where they probably "sell" the account only to you (the "activate" mode). This turns your console into the "main" one for that account, I assume.
Either way, the accounts are at a risk of being banned, or worse, Sony could ban your console. I'm not saying this will happen, but be careful what you pay for: you are not buying a game, you are buying a PSN account, which is against the T&Cs from Sony, so there's no guarantee you'll keep the game very long...