If that guy trades with you its okay but the 650 he wants is crazy XD
a gtx660 really ? the CPU is hardly buyable anywhere any more, just found a used one for ~100$ on Amazon, the Mainboard is nowhere to be found as well and not even an SSD inside ?
I guess all together that thing might be worth 300-400$ at max.
And you have to remember that its used so the chances are not that slim that you might need to buy a new PSU sooner or later
/ I doubt that you could resell it for 500 XD