My daughter was looking for bridesmaid’s dresses several years ago. She wanted rose-gold dresses and wanted to keep the cost reasonable for the women in her bridal party. Searching everywhere, she found some gowns online that fit the bill.
They were relatively inexpensive and they looked fine. As long as you didn't look too closely. These dresses were beautiful in pictures and comfortable but the quality was less than what you would normally purchase. They also left a trail of rose-gold sequins everywhere we went. This was fine because we didn't expect anything more than that from these lovely rose-gold dresses.
This is similar to car parts purchased online (there IS actually a point to this story). People will want to source their own parts because “They can save a lot online,” but you get what you pay for. Especially as it pertains to mechanical car parts. You may wonder why your auto repair facility doesn’t want to install the parts you’ve purchased online. There are several reasons but the main one is; we have to stand behind our work and we take pride in our business and customer service. Our technicians take pride in their work so handing them a part they’re not confident in is something we avoid. If you as a customer purchase an inferior part and we install it, we don’t have any control over quality. If the part fails, you will be disappointed in the whole experience, not just the car part. This isn’t to say that all parts purchased online are lower quality, we just don’t know or trust them and we DO know and trust our manufacturers and suppliers.
If you went to Tory Burch or Coco Chanel and asked them to design you a dress but you would bring your own fabric because it was cheaper, you'd be disappointed with their answer. If they went through with your request, you’d be disappointed with your new garment.