I picked up the Hevnoraak mask while trying to get somewhere unrelated and I think I sold it for sweet, sweet gold! Either that or I stowed it in a container in my house. So I haven't seen it for quite some time.

Now I hear there's a quest or easter egg of sorts that requires that I collect all those masks. I'm just wondering if I there's still a chance that the mask can be bought back from whatever shop I might have dumped it in, or if there's an actual quest that will tag the mask with a waypoint.

Either way, it's cool. I don't have to explore everything first time around.

-K

You're probably out of luck if you sold it. Vendors refreshes their inventory daily, so it might be long gone.
If you're on the PC, you can get it back through opening the console and typing 'player.additem 00061cc1 1', without the ' ' obviously

This is true as far as I know but I ran into an interesting exception once. I sold Belethor in Whiterun a "Staff of Zombies", a non-quest, non-special item and he seems to have it every time I sell him things. It could be a coincidence but I haven't noticed any other item do this and I don't recall him ever selling staves by default.

Without giving too much away, yes. You collect 8 masks in order to the the "Ultimate" 9th one.