This wouldn't be a random site: it could be your bank, an hotel, an events/experiences marketplaces, etc. And there would be a reason for them to offer flights alongside their core experience: it might be they want to enable you to redeem points for travel, have special deals with airlines that they want to pass along to their customers or there's a benefit from a UX standpoint to selling you flights alongside other products.

> Who handles support for rebookings, cancelations, etc.? It's bad enough having to go through an OTA with airlines/hotels when there's a problem; they tell you to call Expedia.

As much as possible we try to enable our merchants to offer self-service flows for cancellations, changes, etc. The airline systems don't always let you do everything programmatically so at that point whoever is selling the flight who be in charge of the customer. We're exploring ways we could provide support ourselves, to take that load off of our customers.