I have bought 2 rounds in the past, both in complete diffrent circumstances.
The first I bought, thankfully was genuine, not that I was to know. I just seen an ad in the paper of a round for sale an hour drive away from me, somewhere where id never heard of. I just handed over £3500 cash in return for a list of addresses. (I dont reccomend doing this, but like I said, it was all 100% genuine)
The second was bought off a good mate of mine, all houses within 10 minutes of me, and also before I went self employed I had worked with him a few times so I knew the majority of customers anyway.
I suggest if your going to buy a round, ask to be intuduced to a selection of customers of your choice, put down a deposit and pay the rest once you have cleaned the whole round. You could even ask to work the round once so that you know all of it.
Hope this helps.