Ebay Parts

Wanted to know everyones thoughts on getting cheap eBay, Alibaba or Amazon parts or geniune parts from the big brands.

I am looking to get frame sliders and other sorts of bike protection but saw that in the branded parts are really expensive like 200-300 AUD while the offbrand parts are less than 100.

Wanted to know peoples experience with offbrand parts and their thoughts on which i should get.