It's all a matter of trust. the way I have done it when buying is to go round with the seller to all the customers, he can tell them he's passing the business over to you, and you meet them. Get a letter from him with his phone number on, to show the ones that were not in when you called round. Then, you pay him, and get round and clean all the windows once (as soon as you can) to 'confirm' it. This will keep anybody out from trying to muscle in. Some people, when I was selling, just wanted the list. The ideal situation is if you know the person selling, and where the round is.