If you have "bought" an account the original owner can take it back or report it hijacked any time they please. You have no ebay buyer protection on this sort of thing. There is also interaction between your main account and any other account you are sharing on your console, which could have security issues.
The bottom line is its something you do with trusted friends not from someone you have never met who is illegally selling accounts (and the game) , which I think under t&c are deemed to be owned by Sony. The players are able to use their accounts by virtue of the game license, its not the same as owning it. Sharing a game is not the same as buying a game. Under copyright laws to sell a game, program etc you have to sell the original disc, you cant make copies to sell and cant keep a copy yourself. Creating an account to share a game and selling it cannot be legal.