How is this homebuilt for gaming?

That GPU and CPU is very outdated and I don't think you can expect even mid settings on games such as BF3. I've watched some game-play videos with that card running BF3 and the person is playing at the LOWEST settings and getting less than 15-20 fps which IMO is pretty crap. It is very cheap and used so I don't know if you still want to go for it but you would be better off either spending more or building your own PC. In other words, this PC is very, very low-end and you definitely won't be able to play any graphic-intensive games of today or even in the past 5-6 years on it.

To put it bluntly, that machine is a piece of s*** built mostly out of obsolete spare parts from 4 or 5 years ago, and with a copy of Windows that's maybe legitimate and maybe not. Anyone saying they built it as a gaming machine within the last 6 months is either lying, or an idiot. I wouldn't pay anything for it.

6 months ago?   he is lying..

Lol I never read that "6 month old" part yet, now I wouldn't recommend it even for $150. You would have better luck buying your parts from your nearest retailer or an online one such as Newegg. It will cost you more than $250, maybe double or more but you will be able to get a much, MUCH better computer. It will be brand-new and reliable. If you really want to game, especially at medium settings or even higher, you can expect to do so with a computer like this, not that $250 piece of ***, out-dated, illegitimate computer you found on Craigslist.