I made the same mistake, but on the XBox 360 version, so no console help for me. Does anyone know if there's an in-game way to get it back? I've noticed that some items I've sold to merchants have remained in their inventories for quite some time, but I've gone around to just about all the merchants I generally sell things to, and none of them have it in their Miscellaneous sections. Do items eventually disappear from the game entirely if you sell them, even important items like the dragon claws?
I'm still on my first playthrough, so it's not the end of the world if I can't complete the quest I need it for, but it is annoying. I don't understand why Bethesda allowed you to lose important, unique items in this game.
Also, just to dovetail on what I said before about the player being able to sell/lose important items for quests that are nonetheless not tagged as "quest items," I just received a quest from Tolfdir, asking me to go to Solitude to close up this rift left behind by the Eye of Magnus, and he specifically says, "I hope you still have the Staff of Magnus lying around somewhere, because I think you're going to need it." So it seems this was a conscious choice on Bethesda's part. Generally, I'm all for games being more realistic, but not when doing so makes them less fun to play. Luckily, I did keep the Staff of Magnus, but if I had sold it, I'd be unable to complete this quest, and it would remain in my quest log forever. Very annoying, Bethesda. Boo-hiss.