Go to a gun auction, you'll never want to buy privately again. Summat like an Omega would be lucky to make £50, and then only if 2 people in the room wanted it bad. Last one I went to I got a 9mm garden gun (no, don't laugh) for £20. It went for so much purely because it was the only 9mm there that had a barrel long enough to qualify for a shotgun certificate. The ones that were a bit short went for 2-5 quid each.
Auction I went to before that I got a Spanish 12 bore sxs for £5. It cost me summat like £7.50 once the commision was added.
ive not seen a garden gun for years come to thin