I am someone who has actually sold on parts of my round.
At one stage my domestic round was large enough that I had 6 guys working for me.
Having had well over a hundred people work for me over the last 20 years (Not at the same time!) I have finally given up on it. Some lasted a day, one lasted 6 years.
I have sold on my domestic round to those that have actually worked for me, so the customers all knew who they were.
But in doing so I wrote out a covering letter for them on my headed notepaper.
Was done on the computer and I simply printed out the requisite number of copies.
My customers also knew in advance (well, many of them did) so were well prepared for when it did happen.
But it is essential that whoever you are buying the round off does something like this.
There may well be the odd customer that takes a dislike to you, but a typical monthly round for one man will have at least 500 customers, as long as you are doing the job properly I would be very surprised if you lost as many as half a dozen of them.
Also, for every customer you lose, and you will always lose some, ie; some move house, die, or perhaps lose their job and cut back on their expenses, you will also be continuously picking up new customers at a greater rate than you will be losing them, so much so that you will end out refusing to take on any more customers.
Any letter written out really does need to be specific for the round you are buying, and it really does need to be from the seller ideally.
Even better is for him to take you around of an evening and introduce you to his customers as he shows you the ropes of the round.
Make sure also that he can be on hand to help you out with any problems you may encounter.
Holding back some of the price for the round is a good idea too, at least until you have done the round for a month or two.
You do afterall need to be sure that the ludicrously large sums of money he has told you that you can earn is justified!
Best of luck.