Because you are trying to trade games which would now retail for $30 second hand these days, for a game which still retails at $99.
Nobody plays NBA 2k7 or Formula One anymore, however Ratchet and Clank was only released a few months ago and is still a popular game. So thus, you are ripping the person off because you are giving them two games of lesser value. Understand?
NBA 2k7 + Formula One = $60 (Also note that nobody even plays these anymore)
Ratchet and Clank = $99
It is just an unfair trade, and I will not sit here and let another fellow myps3 member get ripped off.