It's very easy to know when illegal products are being sold because we as MFR's know who has our products and who doesn't and know where those products were made. Every item from that seller all come from a single source which, as I understand it, no one does business with any longer (or they're foolish if they do). I suspect they are selling overruns, samples, etc... not necessarily producing new figures (though that's possible as well). They can sell them so cheaply because they have ZERO cost. They are in original boxes because extra boxes are provided to factories for drop testing etc...
Basically, if it comes from that seller in the case of First Legion, it's a stolen product, not an authorized/legitimate product. We joined EBAY's VERO program and had our products removed from that seller's ebay listings.
It's crazy, but you can't really stop them from doing it, all you can do is prevent them from getting to the market and depend on the individual collectors not to purchase them. Basically, it's no different than purchasing any stolen good. So sure it's a good deal, buying stolen/unauthorized goods usually is if your personal ethics has no trouble with it and you don't mind stealing (for lack of a better word) from whoever the rightful owner is. Sorry to put it that way as it does sound dramatic, but that's the reality.
Best,
Matt