Yes, for the reasons you just explained. You still need to make your profit and when people bring in their own parts you make less profit.

No, for the reason people buy their own parts in the first place. Its cheaper for them to buy their own parts and get you to install them. If you charge them more two things MAY happen. 1. people will go to other mechanics to get their own parts intalled or 2. people will install their own parts. OR maybe 3. they will suck it up but have a small chip on their shoulder.

Either way its a tough decision. I buy most of my parts from you where either I'm too lazy to go out and do it myself, you get a better deal, or I dont know what to order. The other 10% of the time I'll go out and get the parts becasue I saw a 50% sale on spark plugs, or ebay has an item for much cheaper. Either way, I'm still coming to you to install.