Most purchases of scuba gear on eBay will be completely satisfactory, IMHO.
As far as buying a dive computer on eBay, or any other electronics, look for a good return policy, that is, something like "money back guarantee if item not as described". Also, make sure the item description states the item is
currently working perfectly or is
guaranteed to work perfectly.
Even without such a stated policy, the buyer who pays with PayPal is usually protected very well, but buying from sellers who offer good return policies is often enough to insure you won't need the protection.
I'm an occasional buyer and a frequent seller on eBay and I agree completely with the advice that a buyer should contact the seller and ask questions if
anything is unclear.
Don't assume something that's not written clearly in the item description. As a seller, I'm
very happy to have a customer ask questions prior to purchasing. That helps avoid misunderstandings.
Speaking of buying a dive computer on eBay, I recently purchased one without any problems and, in fact, it was in better condition than I expected.
Prior to the purchase, because I knew what to look for, I asked the seller a few questions and his responses convinced me that the computer was in excellent condition. It was.
In addition to checking a Seller's Feedback reputation, a seller's responses to questions will tell you a great deal about their integrity and service.
Since there are so many good sellers and good deals on eBay, there's no need to go with a seller who's not first-rate.
HTH.
Dave C