Star with arrow in it sounds like it's a Russian SKS made at the Tula arsenal.
Russian makes it worth a few dollars more than $100, if the bore's decent and blue is decent and it's in generally good condition.
All numbers matching also adds some value to the gun. Does it have a bayonet? If not, was the bayo mount ground off? Does it have the cleaning kit in the buttstock and the cleaning rod? All effect value.
The aftermarket stock takes away some of the collector value of the gun but may add personal value to you as a buyer. Many shooters consider the factory wooden stock to be too short and use aftermarket stocks for a longer LOP. The aftermarket may be more comfortable and shootable for you, therefore more valuable to you but not necessarily to the collector market.
Many consider the Russians to be the most balanced and the best shooters but that too is personal preference.
Basically, if it's in decent shape, it's a bargain at $100. I'd buy it RIGHT NOW! Hurry, before the seller figures out he's making a mistake.