Apple Sport bands are made from fluoroelastomer. eBay Sports bands are silicone...very different material and not on the same level for feel, lint-attractability, and durability. I would argue that Sport/Nike bands from Apple are FAR SUPERIOR to anything aftermarket. No debate on this in my mind.
If anything from experience, the 3rd party bands are the ones that are ripping people off...not Apple since Apple does stand behind their quality.
As for the nylon bands...Apple vs. 3rd party, durability will be key here since Apple nylons are more densely packed in from what i read.
As for classic buckles and Hermes replicas, i actually owned several 3rd party leather bands AND one of the Hermes replica. Not even close in terms of leather quality, hardware, and durability. There is some question whether some of the 3rd party leather are even from cow!! If you Google regarding this, then you will be shocked to find where they get leather from to keep prices low (hint: cats and dogs)!!
Milanese is where most 3rd party come closes to Apple Milanese. So, going 3rd party here is a decent option.
For me, i rather spend money ONCE on a quality Apple product rather than spend $$$$ on various 3rd parties that deteriorates in quality. Been there, done that.