But always informed the buyer of such
Supplying an unroadworthy vehicle for use on the road is an offence. A boat trailer is a vehicle for this purpose.Telling the buyer does not mitigate the offence rather compounds it as it proves that you knew it was unroadworthy. Allow him to hitch it onto a car and tow it off and you are most definitely up the creek without a paddle. The offence does not require the seller to be in a trade or business, a private individual can be, and have been, prosecuted. Something to think of when selling on. Writing a disclaimer on the receipt does not help either. If its unroadworthy the buyer would have to collect on a vehicle that could carry, not tow it.
As far as using this one obviously the rig is not legal without brakes but I would hazard a guess that a big proportion of boat trailers on the road and used in saltwater have inoperative/ineffective brakes. Mild steel and seawater are poor companions. A big towing vehicle helps but its still illegal if you get caught by someone who knows anything about trailer law.
Worst case scenario is involved in an accident in which people are hurt. Your risks your decision.