eBay Leaf Spring Kit Any good?

I noticed that people are buying specific parts for our cars and reselling them for sometimes 10 times what it actually cost them.
Examples are,
My gas guage on Ebay was 45 bucks.
Checker Auto has same exact guage for 25

Window track rollers for 15 a piece on ebay.
Autozone has em for 2.89 for a 3 pack.

Wiper arm nylon bushings for 15 on Ebay
Autozone was 3.49 for a 3 pack

They don't even have to buy anything till they get the money since all these items are usually in stock at a local parts house.