When do you inspect a used unit?

I just bought my RV from a private seller. She agreed to let me have it inspected before any money changed hands. We did agree upon price prior to the inspection. That way I knew I could afford the RV before I spent the $250 getting it inspected. She knew I was a serious buyer because I was spending the $250. If any major issues were found, then we would have gone back to deciding who was paying for what with the starting point being our agreed upon price. If we couldn't agree then we each walk away, her losing a sale and me being out the $250 inspection fee. I felt that was money well spent if it kept me from buying an RV with problems. In our case we found no serious issues and completed the sale at the agreed upon price.