How to tell if you're not getting a lemon?

TridenTBoy wrote in post #12454269
A room with a few strands of christmas lights is not a very bright room...

i don't honestly think you can expect top quality photo's in those conditions at f/1.8, 6400ISO & 1/50th especially shooting moving people!!

JWright wrote in post #12448234
You might tell the rest of the story about the shooting in San Diego.

I'm sorry the young man was shot but he made two mistakes. He and his girlfriend (who was uninjured) went into a dicey part of town at 10:30 at night and he tried to follow the suspects after the robbery.

The point is:

1. If you're going to buy off Craig's List, don't meet the seller in a poor part of town at 10:30 at night.
   
2. Report the robbery to the police and let them do their job.

The suspects were caught later that night and are going to be tried as adults...

It's a gamble, sometimes you win, sometimes you lose but you can help improve the odds by doing things smartly. You've stated some of them. But even in the best case there's still a chance you won't find out there is a problem until you really get to put equipment under rigorous testing.