One thing to look for, and forgive me if I am remembering this wrong because it's been well over 30 years.
Way back, again if I remember correctly, the 20 ga Mossberg was constructed with only 1 action bar as compared to the modern Mossberg which has dual action bars.
I had one and shot the %$#@! out of it for about 10 years. There was a skeet range near us and dad reloaded so it was cheap fun. I don't know how many days that gun got a couple of hundred shells run through it by me and my friends.
In all that shooting only 1 thing ever went wrong and that was with the single action bar. Mind you the fact that it had only one bar did not seem to effect how well it worked so long as things were not worn to much. But after a while, all be it a LONG while, the notch in the bar that linked to the bolt carrier wallowed a bit and it gave me some trouble.
Replaced the bar and she went thousands more with never another problem.
So what one thing I would look at, unless the gun seems to not have been shot that much in which case you can bet that it will be a winner, is the notch where the bar catches the carrer. Make sure it's square........
If it's a newer one with dual action bars just buy it as they are very reliable and cheap to fix if you break them.