If you are concerned with excessive top speed, there's a screw in the throttle assembly you can turn to limit max throttle travel. You can easily limit it to only go 30mph max, which is still slower than many regular 50-80cc MB's.
Sell it at their own risk. Make yourself not accountable for their riding. It just happens, that you can get hurt on any bicycle, so they need to be aware of that. If you are smart these can be rode fine, as legal as possible, just don't go too fast in the wrong places. Keep the HP for the highway, and cruise easy in town at nice and quiet low RPM.
It's just as legal as any 80cc bike engine kit would be. Over the displacement and HP, so drive it slow.
IMO, you shouldn't sell it, but keep it as a toy for yourself. If you really must sell it, you could even try to find a cheaper used engine to put in the place of the 6.5, so it won't be so fast, and use the 6.5 for another project. I'm putting a used 2HP briggs on my old trailer so I can sell it with legal HP, geared for 25mph max, and be slower and safe. If you do still sell it with the 6.5, set a good price for it.
Either way, these trailers are cool, many people want one. Few dare to ride with one, hehehehe!