What you have is just another twist on a common tactic: the customer, accustomed to hearing stories about the $400 aspirin at the local hospital, assumes that you're hiding all manner of mark-up in the materials price. So, they get the idea that they can save if they supply the material.
I can't really blame them for trying; after all, all manner of 'consumer experts' suggest this bit of silliness.
I almost think that every contractor should have, printed up and waiting in the truck, a sheet explaining his policies regarding parts and materials. Here are some points that such a flyer needs to address:
First off, YOU are responsible for the job, and any call-backs. You're naturally leery of getting into a dispute later as to whether the call-back was your fault, or the fault of the material. It simply won't do for the customer to supply a replacement part for you to install, for free, later on. You're assuming an additional risk.
Next, there is the responsibility for ALL the materials being present at the right time, and being stored securely. It's no help if you wind up making a return visit, or an additional trip to the supply house, because the customer failed to supply something.
You supply the list? Right - and just how many times have you had to deviate from your first parts list? The customer isn't likely to understand this. Everyone loses when the job grinds to a halt because you need an additional locknut.
Guy works at a parts house? I hate to say this, but there is always the possibility that whatever he wants to supply is stolen, damaged, or returned. Remember: you supply the parts, you control the quality. If you want to use the lever-action Wagos, and the customer wants to foist off on you the cheapest wire nuts, there's going to be an impact on both your efficiency, and another argument brewing.
You should explain that some stuff the customer is probably better off supplying - light fixtures come to mind - because of the very large element of personal taste.
If the guy is 'in the business,' he ought to be aware of the associated costs of maintaining and inventory and handling the parts. If he supplies the stuff, you are still stuck with those costs. That $1 device probably has a real cost to you of $2 - or more. By inserting himself into the issue, the customer is complicating things even more.
Finally, stress that you have your standards, your own 'recipe' for business. Maybe you like to use Brand X and the guy happens to have a box of Brand Y - where does that leave you?
In other words, it's not just about the money, and YOU are not a commosity.
As for the other side of the coin ....
I did a fair amount of work 'following behind' tract contractors, where the new homeowner wanted something different from the 'standard.' While this might seem inefficient, the tract guy was able to operate smoothely - and I had the time to fuss over the details to get it 'just so' for the customer. Overall, I think this was both quicker, and cheaper, than it would have been had I interrupted the work flow of the job site.
