Buying from reputable dealers do not guarantee you'll get the item as described, either. Used gear dealers don't check large amount of inventory they have to a degree we would. It may have been a good unit when they got it but in storage it can deteriorate. In shipping it can get knocked around. A lot can happen to gears. I had many pieces that had to go back for refund or exchanges.

It does guarantee you can return it and get a refund, however....

I have repaired relatively simple lenses like enlargement lenses. But it does take some finesse in doing so. If you are not comfortable with doing this or can risk the lens rendered completely useless as a result, I wouldn't try it. I have destroyed lenses in my attempts as well.