Had a look on Guntrader, and there are quite a few 20G MK60/70 indeed. Most of them in the higher grades as well, surprisingly. No 6000/7000, which are around in 12G. They're not expensive though, so maybe a nice bargain and a 20G set of tubes would work?
I really wish we had anything similar to the UK's S/H market here. We have several shops that carry a good selection of used guns, but it's a small market.
Unless you can actually go to the auction house and fondle what you wish to bid on, to be sure you want it, I'd be pretty wary about anything used. And even then, I would doubt my ability to spot every little thing that could be wrong with a gun... Caveat emptor and so on. If you do end up with a lemon, it's likely expensive to still make lemonade.