He might be selling it for a reason. Do an hour or so of research on WASR's so you know what to look for. I'd start at $380 and work your way from there. They can be decent rifles, but some of them came from Century pretty banged up.
sounds like my century, check if both mags are snug, one of mine is great and the other has a serious case of the wobbles, 350-400 would be good but youre gonna need more ammo, 150 rds flies, and check the action to see if it hangs up or snags esp. if its a century, it can be easily fixed but if its not perfect tell him to knock some off the price
definately check the mags and make sure they fit snug. also check the functionality of the gun and make sure it cycles right. as long as everything seems good I would say 380-430 would be a good price for it.