Classic has K31's listed for $350 and they are sold out. So I would say there is demand and perhaps you could get $400 for the rifle especially since the stock is in great shape and selling with ammo is always a plus. Do you know the date of manufacture?
If you check out youtube or google for anti-tank rifles you'll find some great views and info. I've always been fond of older anti-tank rifles. Those old bolt-action rifles must have hurt.