I found the spectre of Edmon in the mines and took his sword and sold it before I found the real Edmond D'arvias. He now want me to find his sword. Will there be any problems since I guess I cannot buy it back since the stock is always random ?

[Edit]
I summoned the item from the console just to complete the quest. Didn´t think that was cheating since I had the sword just one hour back and sold it because of lack of space and it didn´t seem to be a quest item.

I don't consider this cheating either - it was also the only way for you to solve Edmond's quest. But it's weird you were able to sell it, quest items should be unsellable. On the other hand, this isn't a main quest (if I'm not mistaken) - maybe secondary quests items can be sold...