The problem with the threat you've made is that the person can always claim crap like "I didn't see your thread" or "you didn't give me enough time to respond."
Personally, I would just get it out now, but just tell it like it happened.
"I sold a seat to John, who didn't have the money but needed the seat, so I sent it to him with the promise of him paying me within a month. I reminded him when a month was up, and the following week, but he's not come thru. John, please come thru."