At one of our bigger local gun sellers, I saw a 391 on the used rack and lo and behold it was a 20 gauge. Asked the clerk if I could take it don to look at the gas piston etc. He said sure as long as I could put it back together, The frond cap was seized. I ended up buying it anyway since the price was very good (I suspect they knew the frond cap was seized). Upside down in the safe with a shot of penetrating oil every couple of days. After a week, I was able to get the cap off polish the threads with a wire wheel and could likely sell the gun for double what I paid. I won't but I could. Thank god for some goofs.
I probably would not have made the same choice had I not been so familiar with those shotguns. If a collector gun was dirty, I would insist on a complete teardown to verify condition before paying market. If someone doesn't perform routine maintenance you may be buying someone else's problems.
Regards,
Kris