If you buy a machine from a private person (craigslist, newspaper ad, friend of a friend, some ebay sales, etc) then there are several things you would need to ask about, such as rusty chassis inside (evidence of flooded machine), does bill validator accept modern design bills currently in circulation, are the reel strips cracked or faded, any battery or beverage spill to the mpu board (main circuit board), status of the battery, do keys come with the machine, any warranty on the machine, any cracks in the decorator glass signage, all lights working ok, etc. And you'd want to somehow verify that it works ok, like seeing it in person.
But if you buy a machine from a trusted slot machine seller, like Ken and some others here on NLG, then you don't have to worry too much about the details. You can trust that they will take care of all that.