If the time in which the stereo was "forgotten" was at the time the exclusions were written into the sales contract (or the sales ad), the stereo would be legally conveyed to the purchaser of the car once the buyer and seller complete the sale.
Similarly, once a real estate purchase is completed, all items within the bounds of the property purchased--whether it be roof shingles, trees, or former owner family heirlooms they forgot to take with them--legally become possessions of the new owner unless itemized in a sales contract along with agreed-to stipulations that permit retrieval of the items by the former owner after completion of the sale.
Goodwill (and being a reasonable buyer/seller) goes a long way in situations such as these, and in most situations a buyer and seller will work out property sorting issues amongst themselves. However, there would be nothing legally preventing a buyer that stumbles upon a stash of gold bars left in the basement of their new-to-them house--that they did not know about, and that the previous owner did not disclose--and immediately selling them. [1]
BMW's recent second attempt at "enabling equipment features as a service" is a canary in the coal mine, or trial balloon, so to speak. BMW's argument for heated-seats-as-a-service is, flippantly, "What if the second buyer of the vehicle doesn't want heated seats? They don't have to pay for the service. Problem solved." This distorted-reality C-suite speak so drenched in logic fallacy is worthy of a conversation by itself, but I bring it up to say this: instead of buying a car in the traditional sense, OEMs are attempting to change the model to buying "the physical components comprising a car, and the option to enable features of those physical components".
I want no part of it. I like to use hyperbolic (at least for today) examples adapting commonplace business models for smart home devices, software licenses, hardware compatibility lists, EVs, cloud services, etc. to traditional / legacy / analog items:
- What if you went to use your hammer you've owned for years, only to find out that it can't be used to drive in a nail because the company that made the hammer is no longer in business?
- What if your basement flooded because, while the trench drain around your foundation is physically capable of directing enough water away from your foundation into your sump pump, you didn't opt to pay for the "catastrophic flooding capability" license? Better yet, let's say you paid for the license, but haven't checked your email in a few days (maybe because of the storms, since your power has been out and you don't have ready Internet access) to learn that the credit card the trench-drain-as-a-service company has on file has expired and the license renewal charge was declined, so the license you leased was deactivated via OTA update without notice sent via the post?
- What if you find out while driving that your car brakes won't work because a repair shop you've gone to for years installed a set of third-party brake pads that used to be but are no longer compatible with your car (or part of an OEM-certified or OEM-supported configuration) as the result of a recent firmware update to your car's PCM/ECU?
All of those sound terrifying to me.
[1] Source: myself, but not about gold bars, unfortunately. More than one year post-purchase of a house I purchased, the seller decided they wanted a lamp back that they thought they left at the house prior to its sale. I did not remember if they left it or not, but they did leave several items behind. I sent items to their forwarding address that I deemed to be personal items (e.g. monogrammed clothes), but assumed that all other items were left because they didn't want to take them. I donated all of the items I didn't want, possibly including the lamp they desperately wanted returned, to charities. After spending a good bit of energy harassing me over the lamp, the seller consulted with counsel and learned that they had no options for recourse. The harassment stopped, and I have not heard from them since.