Maybe there's an opportunity here for some kind of buffering service that would receive webhooks and present them as a stream of events. Or maybe something like this already exists?
Can this be a 3rd party service? It certainly can be, but it's hard to make a generic one for any kind of webhook. Some marketplaces expect a dynamic response, like replying the order number we assigned internally (I typically just echo back the number they gave us with some prefix, but it's still more smarts than just replying with empty 200 OK).
And I've seen services which aggregate popular local marketplaces into API which is easier to work with, but they require to concede some other parts of the business we'd rather keep in-house, like assortment and inventory management.
On the one hand, it seems like something too simple to expect people to pay for.
On the other, it's so simple it wouldn't be a huge loss to try it out and see if they will.