Typically software offered at deep discounts on eBay is either (a) educational version; (b) OEM version; or (c) resale copy.
As Zen pointed out, the educational version is usually fully functional but not intended to be used outside academia. This is considered "gray market" unless you're really using it for non-commercial educational purposes.
OEM versions are copies that are only supposed to be sold bundled with new computers or as part of some other promotional package. These are also "gray market" because they're sold in violation of their intened licensed use.
Used, or resold copies actually fall into the gray market, too. If you take the time to read the End User License Agreement (EULA), you don't actually purchase software. You purchase the right to use the software - and you don't have the legal right to transfer that license (or the software) to anyone else.
That being said, it's up the the indivdual's conscience whether or not to deal with gray market software.