The person will sell the house back to the game, so that it is then avaliable for purchase then the possible owner will buy the house off of the game. The amount agreed for purchase is it at some point throughout this process transferred from buyer to seller.
When purchasing guns, a location is agreed to meet and roleplay is had in-game. Player B will transfer X amount of agreed amount to player A, to which player A will give them said amount of guns. Player B will then give the rest of the money over and hope that Player C and D don't rob him of the guns... Whereas player A is hoping player X and Y aren't going to steal his money..