My issue is when I can't get the merchandise when I'm there at the parks, but Ebay sellers have it up for massive markups. If I am actually there, and I can't get the mug I want because ONE PERSON has bought 10, and had their family (including children who have zero interest in the mugs) buy 10 each....that is a problem.
Disney needs to enforce limits on items. Limit merchandise (minus candy/food) to two per person, period, no matter how many times they are in the park. Sure, that means the spouse can buy 2, and each kid can buy 2, but that still means they only get those 6 mugs, instead of the 12 or 18 or 24 (or 30 a day) they would get for going multiple days (or even multiple times in one day). That would mean that 3+ other families could get some as well, using that same situation for them.
And, yes, they can track it, and enforce it, especially the passholders. Target,
Walmart, Kohls, etc. can track my orders and in store purchases made on my account or with my Red Card; Disney can absolutely do the same with APs...and I think they could with MagicBands/Tickets as well. You buy your two, the system locks you out from buying more.
It would take a bit of IT work and some time, but it would work. Sellers could have a bit of stuff to sell, and other people could actually acquire some of the items while they are still at the parks.