screenshots of the tickets in their byutickets.com account, or ask them to call you on the phone while they're transferring the ticket. None of those things are foolproof but might weed out some who are trying to scam. Or I suppose you could ask to meet in person if you live close by.
With that being said though I've never had any issues when I've used KSL, even when I just communicate via text and then send the money.