Assuming you have a cpu and psu to handle either of those cards, your decision should be based on warranty. The r9 390 will be new. It'll be covered under warranty by the retailer and/or AMD for the next few years at least, if anything should happen to the card. The 290x is a great card, Sapphire basically sets the standards to beat for amd cards, but it is used. There'll be no warranty if it dies in 3 months. You'll also have to take into consideration usage. Was it cleaned regularly, taken care of, pushed to the max with OC and run hot regularly, overheated, under volted etc. There may be issues the seller is not forewarning you of, that you may or may not encounter until you duplicate conditions.
The 290x is a crap shoot, the 390...