It's admirable that you're concerned about his towing combination, but in the end what he tows the trailer with and in what manner is not your responsibility. I'm not trying to justify dealers here, but how many times have we heard someone come on the forum asking 'Can I tow this...' or 'My rig doesn't tow my new RV right...' after a salesman has told them 'Sure, you can tow this, you have a V8'?!?!? :S
For your own peace of mind, you can inform the buyer of the overall trailer weight and tongue weight. If the buyer asks 'can my truck tow this' you can help them to understand the GVWR and GCWR of the truck. Once informed, it's up to the buyer to decide if that is acceptable to them. If they are okay with it, sell them the TT.
I have a coworker that was telling me how he pulled his single axle hybrid TT with his new Ram 1500 with 5.7L. His complaint was how much the trailer sways when he pulls it at 80MPH. I 'informed' him that ST rated tires are spec'd for 65MPH and that was most likely the problem. He shrugged his shoulders and continued his search for a WD hitch with sway control to fix the problem.
In this situation I would prefer that he adhere to the ratings, but in the end, how he uses his equipment is his choice and decision. The same needs to be true of your buyer. They can be informed, but ultimately the choice is theirs.
KJ
'14 Ram 2500|Crew Cab Long Bed|4X4|Cummins
Curt Q20 with Ram 5th Wheel Prep
2000 Crownline 205BR
1997 Ranger Comanche 461VS
'01 Polaris Virage TX PWC
'94 Polaris SLT750 PWC
3 Wonderful Sons (21, 15, & 13)
1 forgiving wife!!!