Of course the OP is right. I remember that when I got my first amulet fragment, I was able to sell it. Of course I didn't, because I was on the quest, and realized that I needed to hang on to it. But iirc, I was able to sell if I wanted to. The amulets must be marked as something inbetween a quest item and a regular item.
If you are on a console, the only way to fix this is finding the shopkeeper who you sold it too. I don't know if shopkeepers reset their inventory or not. I think they might get more new stuff after a while. But they might hang on to what you sold them forever. Not sure.
Now that I think of it, there is a theoretical way to fix these issues for consoles.
Is it possible to copy savefiles from a console on a disk or usb-stick ? Or copy files over the network ?
In that case, you could copy a x-box or ps3 savefile to a PC. Then on a PC, you could write an editor to edit savefiles. I looked at the format of savefiles in Morrowind many years ago. And it didn't look complicated. That editor could place/change items in your inventory, or in chests or at shopkeepers. You then place the savefile back on your console. And voila. You get your items.
Anyone wanna create such an editor ?
Would make you famous amongst 4 million console-users.