The thing is corgiman, that:
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Generally, I run my business(es) by arranging to have all the parts I need before arriving at the job, thus avoiding the need to run around like a git in the search for something that will fit. I don't mind paying whatever I'm asked, within reason, because I just pass the cost of materials on to my customers.
However, if your merchants are like mine, each one carries different brands and a different selection of fittings, and I need to visit them all to (a) maintain rapport (and discount!), and (b) get what I want to have I stock.
This is all very well - it's rare that I get caught out without at least one option to solve a problem, but sometimes I do, and, when I do, I need a supplier that (a) won't mess me around and is prepared to help, and (b) hasn't gone out of business because everyone else in the country has switched their allegiance to ScrewYou.com.
[/soapbox]
The second excellent reason for using local merchants is the scenario of problems with materials - I can't be doing with sending things back in the post when they're missing components and/or are faulty. The last bathroom suite from my local merchant had a problem with the pan, and they collected the faulty one and delivered another on the very next day. I wouldn't have got that kind of service from you-know-who.