Make sure it's clean, doesn't have excessive dents, dings etc... If possible have them verify that it holds pressure.
Take a look at the O rings. There should be five total. One large one on the lid, two smaller ones on the posts, and the other two are under the posts, sitting under the pick up tube and gas in tube. If they are excessively worn, just keep in mind that you will have to replace them.
Make sure the gas tube and dip tube are actually in there.
Check the price. If they are charging anything more than about $35, you are probably better off ordering online from a dealer that gives a positive pressure promise.