I bought a car from second – hand store, there was a CD inside the player. The car originally was from a Yid, and the store owner is a Yid. What is the halachah with Hashavas Avaidah, to whom am I supposed to return it. The problem is that the store owner won’t tell me the name of the original car owner, and I don’t trust him to give it back, rather he will just keep it for himself.
If the dealer won’t let you contact the seller, maybe ask him to ask the seller. If he doesn’t want to do that, you have no way of returning it.
It is very possible that the seller care about it, (he might have made other copies of the CD) and he doesn’t really want it back. Besides, he could ask the dealer to get it from you. There is a different issue here- that you might have gotten the CD before the owner knew it was lost. Then you would have to save it, because you are like a shomer on it. These halachos were addressed in the following link. http://dinonline.org/2016/12/12/what-do-i-do-with-a-collection-of-lost-items/