The only fabric I have ever truly had issue with is JAF Keepsake Calico line. They faded horribly and a couple went threadbare on me with minimal usage. I bought these when I very first started quilting and learned my lesson quickly. I will never use that line of fabric ever again.
I have seen knockoffs at WM of brand name stuff (obviously without the brand name on the selvage) and it was pretty obvious it was a knockoff. The prints were way off and the stuff was so full of sizing I bet if you washed it would have no body or hand left to it. A few years back Tula Pink found out some of her stuff had been pirated and blogged about it. Not sure if anything has been done to rectify.
I buy a lot on line too and I do have certain brand name preferences but if I see something I like I order it. I rarely buy full price retail on line, always try to find the bargains and sales, but the occasional cat fabric has had me paying through the nose. Can rarely resist a fun cat print. I have never been disappointed to the point I got rid of the fabric. Sometimes a color wasn't exactly what I thought it would be but I have always been pretty satisfied with the quality.