It's probably pirated if it's being distributed like that. The fact that it's all over the internet means nothing, as MSDN keys and other non-bound keys will have that happen to them five seconds after the product ships.
To be brutally honest, you might as well save your money and pirate it online. While I, for legal reasons, must say I have never done this, and if you decide to do this, I note that it is an action you are taking on your own account and I should not be held responsible, it's probably the second greatest case of Abandonware you can get.
Microsoft doesn't make a penny on ME anymore, as amspire said. While all the EULAs and legal agreements are still in effect, the money it would cost Microsoft to even care for half a second is way more than they would ever see back. It is a bit of a paradox though. While you are DEFINITELY breaking the law, with no sugar coated way of putting it, it's not bothering anybody's profits, and there isn't a single company right now that could really care a whole lot.
A funny instance is WinWorldPC, a site that you can use to download such things. They used to have Windows XP on their site, but were given a C&D by Microsoft to take it off, as they were still selling some versions of XP. They, however, only put a C&D (to my knowledge) for Windows XP, even though they have all versions of windows up to ME on there. That legitimizes nothing, but it is something that shows that Microsoft honestly doesn't care what happens. They've made their dough, and they aren't going to be reasonably making any more.
if you do wish to get a legit copy, buy it boxed. If it doesn't come in at least a jewel case with an authentic looking disc inside, it's probably fake. Expect to pay unreasonable prices for funny people who try to con collectors into buying stuff like that. If it's not too late, tell the people you bought the copy from to keep the board. This is fishy, and you could definitely report that to eBay. What they will do, I don't know.
There are also MSDN subscriptions, but it's WAY more than I would spend (although one would be nice) for something like that.