If you ever need to show someone examples of irony...
Haha.
As a business owner who kind of has to deal with online competition, I can relate. Of course, in auto repair, parts mark-up is not the whole story as it is in retail. But I digress.....
Now, I'm small time with low overhead, so my parts mark-up is minimal anyway-15% on hard parts, 5% on fluids. Basically just covers my credit card fees and shop costs, and what I sell the parts at typically matches the walk-in retail price. Anyway, internet auto parts sales are also huge, and no local store can compete. It's a little different with cars than guns, as waiting 3-5 days for a part is not an option much of the time, but there are still LOTS of cases where people save a good bit on internet parts. Sometimes I even suggest that they go that route, but with a caveat..........
When I buy parts locally under my wholesale account, the distributor is on the hook for whatever period of warranty that part carries. And not only do they cover the defective part, but they also end up paying out on a labor claim for me, should I choose to file one. With this assurance, I can offer my customers a full warranty.
However, if a customer supplies their own parts-online order or otherwise-a defective part is their problem, not mine. If they wanna do battle with the distributor about labor costs, that's their prerogative. But if the part they supplied fails and they want me to replace it again, they will pay full boat labor twice
and have to supply the replacement part up front, deal with getting their money back later.
Now, there are a lot of differences between the business models of an auto repair facility and a gun shop, but the service aspect can be compared. Buy your parts online, I will offer no support for a defective part. Likewise, if you buy your gun online, do not expect the local FFL to be there if you need anything. Conversely, buy your parts through me, have peace of mind that any problem with the repair is covered. Similarly, buy your gun through the LGS and, unless they are complete pricks, they'll help you troubleshoot it and/or send it back to the manufacturer (which they can do for you a lot cheaper than you can on your own if it's a handgun) if there's a problem.
In the end, much of the decision should come down to your service expectations.