Joel,
How used?
Any significant mods to the gun? (Still in warranty)
Does the Sig warranty carry over to the second owner?
Have you shot it?
Does the 'shop' stand behind the gun? (Moot if still in warranty)
Do you know the original owner? And why did he sell/trade it to the 'shop'?
Can you talk to the owner?
Enough questions?
This is what I would look for.