OP, I'm a customer not a tradesman. In the past I've hired tradesmen who were just starting out by themselves - usually just left a site gang and trying to build up their own business so didn't have any pedigree .
If I wasn't sure about giving them a big chunk of cash up front (eg for a boiler) I've asked them to get a quote from their merchant, then I ring up the merchant, pay myself, and get it delivered or give permission for the tradesman to pick it up the morning of the job.
That way the tradesman gets his trade prices and the order correct, and I get title to the goods. It keeps us both happy. It does mean you can't make the same markup on materials so you either have to take it on the chin as part of building your business, or put your labour rate up a bit to compensate.