Are you going to actually use the thing? Do you like,,,,,don't have a shotgun and want one
If so, don't buy a used 870, unless it's a current production 870 Express or Wingmaster, and looks like it's hardly been fired, still has all the blue on the magazine tube and slide rails, and has that aura of newness that just leaps out at you.
Take three hundred dolars to Wal Mart, and they'll sell you a brand new 870 Express with a 28 inch barrel and a laminated wood stock. This is all the gun any of us actually ever need. Nothing will handle much better, shoot much better, or last much longer. It will be new, have a warranty, and you can't get cheated on either condition or price. If you drop it over the side of your boat, you are out less than $300 bucks, so depreciation isn't a valid concern.
I don't practice what I preach. No. I'm a shotgun addict, and I prowl the little gun shops, pawn shops, and gun shows looking for bargains. That means an Express under $200, which doesn't happen, or a Wingmaster under $300, which happens, but only with fixed choke guns, and guns with "issues". I think it's great fun to find a low mileage old shotgun, with really nice metal work, and a scratched up stock, so I can refinish it, put on a nice recoil pad, lengthen the forcing cones, take some choke out of it, add a nice Bradley front bead and mid rib, recut the checkering, and make a really nice old shotgun out of it. Then,,,,,maybe I might get my money back, but the idea is to have a bunch of old shotguns, of all types and makes, as a hobby.
I suppose somebody might shoot an 870 to the point where it's worn out. It would take a lot of shells. You can buy every part that one needs, and so long as one is tight, locks up nice, and the hammer falls when you pull the trigger, it's hard to imagine a very bad shucking buying a used 870.
Just don't think you are going to save any money to amount to anything, buying one. With shells over five bucks a box for the cheapies, you'll shoot up the value of the entire gun in no time at all, if you shoot very much.