I work for a small construction company. We do almost anything that comes our way. Residential mostly, but some commercial. Kitchens & bathrooms, schools & parks. Our hook is that we build custom cabinets in house. As for people buying supplies on their own, sometimes that works out & sometimes not so much.
People used to buy appliances & fixtures thru us. Now they "save money" by finding & buying their stuff online & pay us for the labor only. But, if I sell it to you, I stand behind it. I have some markup in it & that pays for me to swap out a bad toilet or whatever. But if you buy it yourself, I have nothing in the price to cover you if the thing is bad. You'll have to pay for any additional work.
I recently installed a brand new bad toilet that the guy bought himself from the wholesale house. It would leak from under the base after 4 or 5 flushes. It looked like a bad wax ring at first (which would be my fault). But after setting it for the 3rd time I took it back to our shop, set it up on sawhorses so I could see under it, filled the tank & gave it a flush. It worked perfectly. For 3 flushes. But on the 4th flush the unseen void in the casting filled up & it started to leak up front (which was not my fault). The seller swapped it out easily when I showed him the video. But the homeowner paid for all the extra labor done that day. I don't know if he got any money back from the seller.
We had a lady who bought a built in closeout fridge & freezer (sub-zero style) online. They were designed to hold matching cabinet doors on the front. But the 2 machines were of different generations & the frames were different. There was no way for us to make them look alike. Even though she screamed at us about it. She had to chase down a replacement & deal with getting rid of the wrong unit. I have no idea on how much she "saved" by the time the dust settled. She screamed at us several times throughout that job. Free work that she wanted us to do that we wouldn't do & so on. She also wanted us to do more work after we finished our contract, but we passed on taking on anything new.
We had another customer who figured out who our window supplier was. She went directly to the supplier, lied, & told them she worked for us. And proceeded to order her own windows to cut out our markup. When you order windows, you give the width first & the height second. But if you are trying to cut out your contractor you might not know that. When her windows all arrived wrong, she called & wanted us to help force the supplier to replace them, since they didn't explain the industry standard to our "employee" when she placed the order. Uhhh... no.
Dishwashers burnt out by new garbage disposals installed without knocking out the plug. Microwave hoods installed at such an angle that liquid won't stay in a bowl. Fences built with the lumber installed wrong & sagging. Officials angry & plans "lost" due to bad behavior at the counter. Copper pipe not reamed & pinholes showing up years later. Wire run just below the surface of stucco burning when a shelf was hung. The same stupid wiring trick in a bathroom, with foil wallpaper, & a towel bar screw connecting the foil to the wire, lighting up anyone who touched wallpaper & the faucet. They are all going to save money. And we continue to get calls to undo whatever mess has been created.