Is there any evidence that eBay actually received any benefit from the former company's patent, rather than having developed the ideas independently?
In theory, the patent system was designed so that people would patent their ideas in useful fashion and, as a reward for doing so, receive temporary exclusive rights to them. In practice, I think the primary effect of publishing patents is to prevent any sort of "independent derivation" defense, even in cases where it would and should be applicable.
Although there are times when it can be useful to look through expired patents to see how earlier products work (e.g. Activision's Pitfall II cartridge for the Atari 2600), I don't think very many people look through current patents for new ideas. While I don't know about this particular case, it's not uncommon for some companies to patent ideas they have no idea how to do anything useful with but which might become obvious due to technological developments. When the ideas become obvious, the "inventor" then gets to go after anyone who independenly comes up with the same idea.