For a used double shotgun, the buyer’s equivalent of kicking the tires is to jiggle the action to see if it’s still tight. Take the grip in one hand, the barrels in the other, and wiggle up and down, checking for any looseness or play. Be sure to remove the gun’s forearm first; an action that seems tight when you shake it with the forearm on may rattle like a flamenco dancer’s castanets when you try it with the forearm off.
A loose action doesn’t mean a gun is ready for the junkyard. Tightening an action or putting a gun “back on face” usually costs $300 to $350 and will make a gun lock up like new. –P.B.