this is such a wide open question because unlike buying from an online quilt shop, there are about a zillion different sellers. Having said that, I have bought LOTS of stuff on ebay including fabric. I look at the feedback number, and if you click on the number, you can actually see the feedback posts. i look to see what items the comments are about, then make a decision. There is a place on this board where board members sell fabric. This might be something you would also like to consider.
http://www.quiltingboard.com/items-sale-trade-f20/