Some people on Ebay buy the parts in bulk quantity and sell them cheaper.

Some are also selling the same quality parts at resale cost or 10% above cost to make thier sales quota with a particular vendor.

I recall from when I was in the shop that some parts have a 200% markup on them. I remember one Alternator I did where our cost on the unit was 80.00 and the MSRP was just under 400.00.

Its not always that the parts are lower quality.

Terry