If you are not making profit on the parts you should make it up in the price, you are not supposed to get hurt here but bear in mind you should only be marking up the profit, minus the money you didn't spend getting the parts in the first place.
There is a difference between markup and profit.
Of course using the customer's parts comes with it's own pitfalls. If they don't bring you the right stuff your labor is higher and you will still need to get the right thing on the clock so the saving for them may not really be there.
The reality is, they may actually be saving you a lot of hassle by spending the day at the BORG, assuming they get the right parts.
If I was a customer I might shop around for the big ticket items that you would have to special order anyway (like a big genset).
It might even be a personal preference for a brand your distributor doesn't handle.
I wouldn't want to fool with the standard stuff you carry, even if I could save a few cents per wirenut or a dime a foot on the wire. It would be too easy for my job to grind to a halt because I forgot something or measured wrong.
