For 30 plus years I bought a ton of orphan barrels in common American doubles. Fox in graded, Sterlingworth and small bores, Baker, Lefever and many others. Often barrels in good condition could be bought for $20-100.00. Wish I could find a nice set of 20 Sterlingworth barrels like that these days. I’d even go as high as $200.00. Big spender.
Always hoped to find the rest of the gun. Did that exactly once. Bought the barrels first, forend with metal a few months later at a gun show in the next state. Then bumped into the first dealer who had found the rest of the gun while cleaning out his shop. Funny thing was he could not recall what happened to the fore end. Wear did not match, so I figured that fore end got used on a second gun for years.
I fit many a second set of barrels to a gun over the years. Sometimes they almost fit themselves and sometimes they took a lot of effort. Smoke, files and patience go along ways in fitting them. I tried magic marker and even lipstick as a marking system. I am down to just a few orphans these days and have little interest in buying more. One big factor is having either the proper forend for your spare barrels that comes with them or an extra one to fit onto the second set of barrels. Too many times the barrels would work but required fitting the forend to either both barrels or hoping you could get by until your found a second forend.