marbehshalom,
To answer your question about the buyer not seeing it on the cover, I would say that it might depend on where you bought it and the arrangement with Aderet. If it was bought at Eichler’s (just using the example you used) and, in fact, Eichler’s actually rented it, it probably could not be subsequently sold. If it was bought at Mostly Music which is owned by Aderet, then it’s an interesting shaila, which I think would require the shikul hadaas of a dayan to determine if it’s reasonable to expect the buyer to either see it on the jacket, or know that it’s common for CDs to be rented.
If it’s determined that the buyer was not expected to know, then, back to Eichler’s, it would be the store’s fault for not explaining to the customer that it’s a rental. Then another determination might have to be made if the difference is significant enough to render the transaction a mekach taos, allowing the customer to return it for a refund if he so desires.