I've purchased used parts from CF and from ebay and brought them to my #1 shop for installation. Frequently, the installation will involve additional parts such as cable, housing or some other part that I didn't think of. I buy those from the shop that's doing the installation. They've never complained. Hey, I'm still paying their standard labor rate for all the work they perform and I still buy some parts and accessories from them. Also, I never ask for a discount and never complain about their prices, although I do ask for an estimate before they start the work. I can understand what the OP is saying about someone who just abuses the bike shop, but some shops won't work on a bike if you don't buy the parts from them, especially used parts. Others, like the shop I frequent, understand that when a major installation occurs, there will be accessories or associated parts that must be purchased, plus the job pays for salaries.