I'd say Belgianpup is right on the mark and that this Arc is not as bright as it should be and was therefore sold on Ebay.
I can't think of any good other reason to sell such a great light!
There could be lots of reasons it was sold. One thing to note is that I've heard Arc had some mfg issue and had a large gap in mfg. This caused their to be quite a shortage of Arc AAAs. That, combined with how the seller purchased it (I've picked up Arc AAAs as low as $15, which is below the mfg cost I understand!), might mean they actually got it a lot cheaper than retail, and in a time of high demand and low supply finds this an easy way to make some cash.
Other reasons are a lot of folks "horded" these lights. People tend to buy them to give them as gifts, etc. So it could just be leftovers.
There are also different vintages of these lights, with constaint improvements. So, someone could be replacing their older stock with the newer ones. However, they all were pretty darn good lights.
There are also some less noble possibilities. One possibility is it is a "second" (Arc has sold their "seconds" with full disclosure, but that doesn't everyone will - I don't know if they are marked in some way), or not a premium at all. Note that the premium is marked on the body (anyone know if there is any markings on the head?), so someone could potentially swap the head for a non-P.
Picture from Arc websiteWhich of course doesn't mean this light isn't a current generation, first quality Arc AAA Premium, it is just that there are a lot of possibilities.
Note the Premium is currently selling for $34.95 on the Arc website.
No matter what, this is a great light. Don't sweat it!
-john