Baggins: It actually sold more than previous games initially. However, I think they overprinted them, Vivendi was trying to sell copies for years after the game's release. So many copies still out there. There are still stores dispatching new (unsold stock) copies as well, through normal online main channels (for example Amazon still sells them from their main store front through an affiliate). Not counting secondary market of people who did try to get rid of copies for whatever reasons.
Actually pretty much all the cd based games are cheap now, so many on used markert, and CDs last a long time (unlike) old floppies which are much rarer. I found KQ6 for a cent on amazon!
Now with GoG there is probably even less demand for physical copies.
The new installer btw apparently uses NGlide which as mentioned was a terrible choice.
I worked at Target when Mask of Eternity came out and remember the price falling quickly. I worked in electronics at the time and know we sold a good amount of copies, but there were always more that would arrive on the truck. I ended up buying the game for $15 only about 4 months after it came out - and that wasn't with an employee discount.
It actually was the #2 selling game in the department around Christmas time, with Ocarina of Time on the N64 being the champ. We just had way to many of them. A shame, really, because there was a market.
MoE wasn't exactly a traditional adventure game, but it's a lot easier to make money on a smaller market nowadays thanks to digital distribution. Running a business where you have to try to figure out how much media to press, trying to thread the needle so you don't have shortages but you don't have much excess is difficult. With Steam, GOG, and the stores run by MS, Apple, Google, and Amazon, it is a very different environment. I'd love to see a revival on modern tablet devices.