Many of the rifles of that era are rebuilds.
Yes...the only thing I can say is if you see a vibrated engraver number on it or even a laser-dotted SN, it is likely not worth the $500 price tag.
Of all the Russian/Yugo imports, I think the SKS is the least valuable. I used to see these things go for $100 over a decade ago. Heck, I once bought 6 of the folding bayonets for a dollar each to use as tent stakes (work great for that).