I’m not a lawyer but I’d bet if you allow him to use the trailer and something catastrophic happens you’ll subject yourself to potentially being held legally liable.
Have the buyer rent a Uhaul single axle trailer (rear axle of 40 is on the ground) or use a Uhaul 2 axle trailer. And have them get insurance.
Why you ask? I bought a 45 bed in NV in 2020 and used a Uhaul 2 axle car trailer to haul it back to VA. I was driving it back on I-20 in LA to pickup a 45 I had purchased in MS and I was unable (car in lane beside me) to swerve and miss the metal pipe from an Interstate sign in the middle of my lane on I-20. It also got a charter bus that was able to thankfully stop before hitting me.
It broke one trailer axle and blew 2 tires and destroyed both rims. I drug the trailer off the off ramp and parked it at a grocery store. I drove to the Uhaul location got a new trailer moved the bed over and continued on. I paid nothing for any of the damages other than the insurance. Best $90 I’ve ever spent. And it was the first time I’ve elected to get the insurance.