Having bought many rounds over the years, some are worth paying good money for, some are not, you need to see he quality of customer, pricing, and very important to look at the HISTORY of the customers. What you don't want is a £30 house sold as being done monthly, but really, they only actually get it done 3 times a year due to access, holidays, rain excuses etc etc.
You also should look at payment history, ad again, do you really want a load of houses that don't pay you or weeks on end?
You also need to be realistic and view buying rounds with "people will be people" attitude, as you will get cancellations, people trying it on, people not liking your approach etc, so be prepared to lose a percentage of work over the first few months.
I don't pay for work before its been done. A deposit followed by instalments, minus cancellations is the best way I have found as it is fair to both parties. There are people who will pay all up font or a load of names and addresses. Buyer beware.