When I sold my round I met the bloke on the round and introduced him to each customer or if they were out I left a note explaining the changeover with both sets of contact details. Then he paid. Obviously I gave a list with details.
I think that's what I would expect as a minimum if buying work.
Dont part with loads of cash upfront there are scammers out there I know a guy who bought work and the streets didn't even exist, let alone the work