Two options, both of which seem to work well in venues near me:

1. When an event sells out you can join the 'waitlist' and people can offer their tickets back to the ticket company who give the person at the top of the waitlist the opportunity to purchase. All at face value. Good for the artist too as there is less chance of empty seats when people can't make it.

2. QR code tickets that rotate meaning they can't be screenshotted and sold.