One common characteristic of many counterfeits is that they lack tagging. They also frequently lack microprinting. Additionally, some counterfeits that I have seen have suspect die cutting especially obvious where the vertical and horizontal rows meet. Genuine stamps usually have clean intersections. Here's a counterfeit example.
Obviously, tagging can't be checked before you buy. As for checking the other characteristics, the scans that the seller posts on eBay
may be of genuine stamps.