Not rich, just devoted.

The $500 price tag is pretty ridiculous. Someone recently bought a copy of the game at that price, so I guess there are some people willing to pay that much.

I have a sealed copy of Keen 6, but it has some sentimental value, and I'm not sure a fair price to ask for it, anyway.

I don't know about that. There's way too many holes in that for me to buy into. For instance, if it is Keen 6, then why on Earth doesn't it say so on the box cover? All the other keen episodes in the series have numbers to denote that they're apart of the same series. Aliens Ate My Babysitter was also not published the way that the other games go.

Just seems to me to be a bit fishy that iD would handle it like that.

Absolutely. I also prefer to actually own a copy of a game, as opposed to just "having" it in the form of some impersonal file.

None of the FormGen releases have episode numbers on the box, but the in-game story clearly states that Aliens Ate My Baby Sitter! is Keen 6. You can see it for yourself at 0:38 in this video: http://youtu.be/AnhcTXXdD7M?t=38s.

I agree that it's weird that it was a commercial release when all of the other ones were shareware, but that's the way it is.

Actually, the AAMB is connected by plot to the others. In fact it ends on a cliffhanger of sorts.

Is the lack of number so important? Look at Might and Magic 4 & 5. They don't have numbers either. They were called Might and Magic: Clouds of Xeen and Might and Magic: Darkside of Xeen without any numbers. Does it mean they're not canon? 3DO called the next game 6, and recent release (like the one on GOG) do include the numbers 4 and 5, but you'll never find them in the game itself.