Not all dealers have the same quality when shopping a game. I have seen many privately shopped pinballs that were way nicer than some dealer pins. And it doesn't matter how big or fancy the dealers showroom is. That doesn't necessarily reflect how good he or his service guys are.

Also, not all dealers warrantees are useful. I have heard of dealers who sell a game but then won't come out to service it or just put you off time and time again.

Unless you are a total newbie who needs delivery, set up, and after the sale service, I would say just evaluate price and condition no matter who is selling the pin. Dealers will almost always charge significantly more than private sellers. They have to pay their overhead and employees.

If you are new enough that you might not know what to look for on a pin, you can get dazzled by all of the bright lights and sounds.
Try to take a more experienced collector with you to point out little details you might miss like faded cabinets, cracked plastics or ramps, damaged battery holders, burned up connectors, playfield wear, dirt under plastics, etc. These can significantly affect game value and can help separate a truly shopped game from one that just got a quick once over.