Questions to ask.

There is a strong possibility that I m at be in the market for a pre-owned deck oven. One in particular that I stumbled across yesterday, a Revent 3 deck oven, it looks good in the pictures but before I go take a look at asking questions about it is probably the first step. I have never done this before so I'm not sure what all good questions should be asked. So with the very knowledgeable community here I'd appreciate your suggestions on what I should be asking in order to make a well informed decision. Thanks.