Well, it goes without saying, but authenticity is pretty important, if you're going through eBay.
I look for one that size/shape-wise I know will look good on me (so I confine my search to only certain styles). Then I have colors in mind, so that if one comes along in a style and color combination that I want, then I can seriously consider it, and for me the price has to be reasonable.
Some are very picky about what kind of leather they want. This is hard to explain unless you have experienced Balenciaga leather at any length, but personally I take each bag on its own merits and I have loved bags in the past that were very veiny, as well as very smooth bags.
I think it's pretty important to do it that way because of the many styles and colors Balenciaga has to offer! When I started out, I went to a store I knew had a good Balenciaga selection and looked at many styles and colors to really get an idea of what I was dealing with (btw, from that point on I was hooked!).
Good luck!