IIRC you couldn't sell to the person who was marked (behind the scenes) as the object's owner, but the object wasn't classes as "stolen" and made impossible to sell to anyone but a fence, as it would be in later games.
He might be thinking of Daggerfall, where you could use the "wait" command in a shop until the shop closed, then take all the items off the shelves and none of them were considered stolen.
You could if you sold items from one vendor to another vendor, e.g: from another town.
Yes but in the long-term this screwed you hard. If you ever stole e.g. a diamond from one trader you could no longer sell that trader diamonds. Even after years and dozens of looted diamonds you still couldn't sell diamonds there.