Like most companies that allows warranty transfers, it'll have to remain in original condition (all labels, stickers) and have the original receipt. If these conditions aren't met, you can't count on the 9.5 year warranty being a thing.
How good a deal depends on how good a deal it is, which means model and price. I personally avoid used PSUs; it's safety equipment. But if it's a choice between a used quality PSU and a new junk PSU, I'll take the former.
Personally I wouldn't buy refurbished PSUs
Its very much like sharing needles for vaccines.
You can never ascertain the state of quality it is in or how much useful life it has left.
If thinking environmentally wise, PSUs inevitably get recycled to have their materials reused in another way.