#12, Check out the sellers return policy ,if it says something like  "I know what I am doing and you better too or else",
RUN !
here is the great "it works for me" return policy
Here is a good example:  
Ed Goods Bogus Return Policy   "This was posted recently on another forum that I browse.
"posted 12 December 2013 23:52
If you're into older SxS shotguns, I have a head's up.
 Ed Good in NH is an appraiser, and lists many older classic SxS shotguns on Gun Broker. I bought a vintage SxS from this company, but when I mounted the barrels, they were loose. I tried to return it, but was told that since the action is tight when the forend is attached, it is "shootable", and no return would be accepted. The forend is a part of the action's lock-up (???). I sent him several articles on checking a SxS for wear looseness just as I had done, and he said that was a matter of opinion. I wouldn't be surprised if most of his listings are beater guns, waiting for sale with no chance of return, to become someone else's headache. Steer clear.