The best advice I've heard for what to auction is to check are all three of these things true?

- Low supply (few others for sale on eBay)

- High demand (what people definitely are looking for) and

- Unknown value (not able to search Completed listings to get an idea of what people will pay).

Otherwise Buy-it-Now is best. eBay used to be known as an auction site but that has changed in recent years.