We've had a couple of people come in and ask us to install used parts. We don't mind that. We just do it and charge them appropriately. Lot's fo people swap around parts. It's no big deal.
I also don't mind wally world bikes. They are just another service item.

What would get to me is if someone came in with a brand new component in the box and said "yeah I got a steal on this at price point. Can you install it". Mostly because that person didn't even give us a chance. We would still do it, but we would charge them full pop on the install, and in the end it would probably wipe out (or put a big dent) in their "savings". Now if they had bought it from us we would have most likely installed it for free.

In the end the we are a "full service" shop, meaning we do just about everything. It's part of our business model, so we don't think twice about it.
But as a brick and mortar owner it's really hard to be constantly hearing about how cheap things can be picked up on the net. We work hard to keep our prices low, but we have overhead. And the thing with giving the lbs the business is that they will be there to install it for you. Those places on the net don't have mechanics on staff waiting for you to bring them your shiny steed.