[mwgalloway]

Using used parts might be twice as much labor for one thing, you have to remove the part carefully from one phone and then install it into another one. Another is that you can't always guarantee the repair is done well if it's done with used parts as you can't really be certain what kind of conditions they were subjected to over time. Also, as you have to charge less for used parts, you might not have as good of a margin on the repair. I don't work in phone repair, but I used to work as a bicycle mechanic, and sourcing used parts for repairs was generally not considered worthwhile in most shops I've worked in.