If he were willing to sell it to you for $500, I'd have to ask why. Firearms aren't like used cars. They really don't depreciate. He could probably get $800 easily and definitely could sell it to a dealer for $700 or so. What he has with is affects price as well. Cases, optics, etc can add alot in value.
You may see it listed for that lowball price, but actually getting one could be a whole different story. The reputable one's are selling for just shy of $900. and remember with mailorder you have shipping and FFL transfer. If you don't have someone to do that for you, you may have trouble. More and more dealers are shying away from doing those transfers.