The concern is not about if a seller can sell something they've legitimately obtained for less than it cost them to obtain it. The concern is about whether the product was legitimately obtained? If Apple, for whatever reason, deems a particular Apple Care entitlement was not legitimately obtained from them then they could, and likely would, refuse to honor it. A second party buyer may not learn there is a problem until they attempt to use it which could be well after the sale.
There are legitimate sales and there are illegitimate ones. Only a buyer can make the decision if the risk is worth the lower cost.
That's exactly why I kept it a simple, safe but costly AC from the MAS!
~ Cheers