Sears does have some cheap parts. However, I agree. I charge customer retail price and I still make money buying at my local parts place. I like going to parts place, picking up part, and having it same or next day. If I have a problem, I bring it back and get another one. I've had customers who make a stink order the part themselves and then I'll put it on. I tell them the problem with sears or other online sights, you have no idea when part will arrive and some take a long time. That's why I don't order from them.
However...........what I do is when they ask me how much to put it on, I tell them. Then I also inform them that if they do want to order part themselves, I'll put it on but no warranty. If part goes bad within 3 months, then I will charge them another service call and labor to remove and install new part. They usually have me get the part.