I agree with the above comments, particularly Glenn's (tongue-in-cheek) termites thought. To put it differently, I think you can divide the questions into two categories:
1. camera features, and
2. camera condition.
By doing research in advance, you'll know whether the camera will do the job you need. Past threads here on APUG delve into the features of various cameras and the suitability of those features for various tasks. Additionally, there are a number of articles on the
Large Format Photography Home Page that may be helpful in choosing the best fit between your objectives and a specific camera. Once that choice has been made, or you've arrived at a short list of several candidate cameras, you can get serious with sellers.
Questions for sellers might include:
1. has it been dropped or otherwise damaged?
2. are the standards still rigid and tight when locked down?
3. are the bellows in good condition and tested for light leaks?
4. is the ground glass in good condition?
5. if the ground glass is not original, what brand is it, and has it been tested for positional accuracy?
6. are the springs on the back still tight?
7. is the gearing still good?
8. what is the seller's policy on returns, should you find the camera to be unsuited to your needs, or not as advertised?