I used to think that way. However after many garage sales I have had, you realize that it is how some families shop for new clothes. Goodwill is not "cheap" and frankly the selection is poor for "teen clothes".
I never forget this family with preteen girls that were so happy at the clothes I had for them. I was selling everything 10cents just to get rid of it and it was good stuff.
I had people buying armloads of clothes, dresses, and coats.
I had a family that bought all my coats to donate to their church.
Yea, some probably bought and sold it to resell it shops to make money, but you know what, maybe they need that money. For some people that is a part time income.
You can look at it anyway you want really.