I think that model is "post-ban"/commercial to the hilt-Monte Carlo stock, no bayo, bizarre magwell config., etc. I'm not a big fan of SKSs that take AK mags, but if I did get one, I would want it to be a D, not an M. $500 may be what they go for, but I think there are better options out there for 7.62x39 rifles for the money right now. A $529 N-PAP, or a Saiga, come to mind.
The seller said he's tried quite a few magazines in it and only chinese 30 rounders work reliably. That pushed me off. It's very clean, extremely clean and doesn't look like it's been shot much but, unless I can get it for much much cheaper I don't think I could justify it over just buying another Saiga or something.