I tried to buy AppleCare+ without linking it to my AppleID but the rep on the phone wasn't able to do it. So, if I want to sell my Mac or iPad with AppleCare+, how will the new owner get AppleCare+ if it's associated with my AppleID?
If you buy the "pre-paid" AC+ for a device, then I believe the warranty follows the device. But only for a set amount of time you pay for up front. If you use the AppleOne care plan or the AC+ plan where you pay monthly, then that plan is tied to your Apple ID and can't be transferred to the new owner if you sell the device. You drop the plan for that device when you sell it.