I may be splitting hairs, but in this, there's a distinction between the government making it illegal to resell items and eBay caving to a letter from MicroSoft lawyers. A common tactic with large companies is to demand compliance on threat of a lawsuit. eBay makes little off of individual sales, so it's easier to comply with a letter demanding certain actions from a MS lawyer than to enter into a protracted legal battle with a corporation that has limitless pockets. MS has also, in the past, demanded full access to all computers inside a business to see if their software is being pirated. They have no legal authority to do this, but can bankrupt almost any small to medium business by simply filing lawsuits.
The fact that eBay complied with a demand from Microsoft would have little bearing on whether it was actually legal to resell the software. FWIW, Disney used to try and claim it was illegal for places like Blockbuster to rent their movies.
Good points, and I agree with you.