I have never brought a round, i was given one, which turned out as next to useless and all the customers were well underpriced, I have 2 from 30.
If I was to be tempted into buying one though, I would need to know a few facts
How long has the cleaner been cleaning them, if less than 6 months, be careful as they might still cancel.
What does he charge? if he is charging the same or more than you, then great, its also ok if he is £1 cheaper because you can get this back with your yearly price rise, but if he is £££ cheaper then don't touch, you will lose loads.
What system is he using? although most people accept wfp cleaning, there are still a few who prefer the old ways, these will move when they can get a trad cleaner.
I would pay him a percentage up front then I would insist on going with him on his last clean, to be indroduced as the new cleaner and if all say they are happy with the arrangement, then make the final payment.
and finally i would have a agreement drawn up that offers a money back if a certain percentage of his customers cancel within 3 months of you cleaning them, I would also add to the agreement that he cannot canvess or solicit any of his previous customers or area he worked in.