If you pay a higher price, you will have some sort of warranty. That’s why they charge more per hour.
And they have to mark up the parts as well. How much? Every contractor has their own rules. If they buy $300 worth materials at HD, they should charge you at least $400. Maybe $600. Why? If the material doesn’t work out, is defective or what not, they need to get replacement. It’s eating up time. I’ve bought brand-new breakers from HD in the box (not returned) and they would not work. Don’t get me started on all the packaging that looks OK but someone skillfully stole (essential) stuff from it.