Worst part of the pc would be the motherboard being very bland, gpu is kind of older, and power supply don't know which model within corsair it is. But all that said, motherboard bland yes works yes, gpu older yes good yes, power supply its corsair their budget aren't as good as mid or high end but still good.
So overall, id say you got a good deal. Its used, yes, but so was your xbox. the i7 goes for $300, motherboard idk like $70, memory $70, hhd $120, gpu $200 ish, psu $50, dvd $20, case idk $75, cooler idk $30, controller idk $20, windows $100. Add all that up, about $1000. Used, means its really up to the buyer/seller how much theyre willing to spend vs how much willing to let it go for. But xbox $500, $200 $200, so $700. That said $700 wouldn't allow you to build a build like this, and it should play games high/maybe ultra. (which is better than xbox). SO overall, I think you both walked out pretty good. You have a pc you can game on plus do almost everything/anything else, he mustve wanted an xbox, so got one plus $200 for games controllers ect. SO, win win!