I have purchased fabric on e-bay. I look for words like quilt store quality or specific names like Moda, RJR, Timeless Treasures to help me decide. I have found that the more cutsy the fabric, more likely it is not of high quality. I have received poorer quality in the past. I am much more careful now. And you get good deals there.
That said I usually buy my fabric at LQS or through Connecting Threads. I spend more, but I get what I want instead of what is being offered.