Was the argument about the properties, or a personal argument?
No need to say you fell out because he supports City and you support United, for example.
But if it is a property issue that affects the buyer, then it should be mentioned.
If you're talking about an argument purely between you and your neighbour then the answer is no. However, if any other parties were involved (i.e. the police, local council, solicitors, mediation etc) then you do have to declare it. If you don't declare it and the new house owner is in dispute with the same neighbour or finds out you haven't declared a dispute, you could have the pants sued off you.