Assuming it is not really beat up, it is probably worth a minimum of $1,500. If it has no significant splits or breaks in the buttstock or forestock, and the stock has never been sanded down and refinished, and the metal has not been "cleaned" by somebody taking sandpaper or steel wool to it, then it is easily worth about $2,500.
If, however, the wood has been sanded and refinished, or the metal has "cleaned" by buffing or sanding, or the metal has been re-blued, then no matter how nice it looks, it is knocked back down into the $1,500 range.
If you buy this rifle, then DO NOT clean, sand, buff, steel wool, or refinish it in any manner. With antiques, the old "patina" is virtually everthing, and if you clean this off, you will cut the value of the antique in HALF instantly!
The most you should do is take a clean, soft, cotton rag, and put just a small amount of lightweight gun oil on it (synthetic like Rem-Oil is best), and just lightly wipe it down .
Hope this helps.
Mannyrock