I'll be interested in seeing what others have to say, but I suspect the seller may be closer to reality than you are. Within the market range, value depends on condition, condition, and condition. You haven't told us much, other than that it needs a stock and a rear sight - maybe $100 in the current market. Pictures would help. From what I've seen, $875 is probably in the general ballpark for a good USGI shooter.
"They've took the fun out of running the race. You never see a campfire anywhere. There's never any time for visiting." - Joe Redington Sr., 1997