It may be easier to tackle the problem from the opposite end: why are people going to these resellers, and sometimes paying higher prices, to get these items? It would take away the exclusivity of items, but if more park merchandise was sold online through official sources, we probably would have less of an issue. For instance, if I'm looking for Tower of Terror merchandise, the only thing specific to it on shopDisney.com (currently) is a Funko Pop set. I know more Tower products exist. If I was not able to take a trip to Disney, let alone solely to purchase an item I want, then of course, I am going to look for other ways to get it even it means paying someone a bit more to get it for me whether I know them or not.
The exclusivity could be there with the advertising that it will also appear on the shopDisney website within a certain time frame. I'm sure the exclusivity gives them the ability to mark up products higher, however, I'm curious if the online sales from across the world would offset it at some point.
That's what I would imagine would cut down on the incentive for locals to go put that much time and effort into it if it won't get them the profits they were getting.