I dunno, non-refurb with all matching numbers, if it's in good shape, could be worth a bit more than $350. If you look on Gunbroker most of the Russians are closing between $400 and $600. Hard to believe for those of us who bought an SKS back in the 90's (I got my '51 Tula for $150 I think).
My understanding is most SKS's have been refurbished at the armory at some point (mine, for example, has a non-matching serial number on the magazine) which obviously lowers the value.
I'd say if you're getting it for $450 or below you're probably doing well. If you can get him to throw the ammo in for below $500 I wouldn't feel too bad.