The business revolves around bedbanks contracting hotels and selling them to retailers and aggregators. There are exchanges, there are hybrid models and what not. That's why "2 rooms left" kind if messages are almost always total BS because every hotel can have more than one bedbank contract or the contract can be flexible and you can't know exactly how many rooms are left but you can query if the number of room you want to book is doable.

At the end of the day, what you have is multiple networks that somewhat overlap and provide API for finding and booking and Web interfaces for the high street retailers etc.

Making an aggregator means, you getting an access to these API and therefore there are many aggregators out there. The problem is, the margins are really tight at that point and you can hardly do anything about it. The money used to be in the high street retail where you can have fat margins and sell extra services like city tours, paragliding - that also have fat margins.

The industry have seen quite a bit of squeeze both because of AirBnB and the aggregators.

A good way to make a lot of money is to contract all the hotels in an area and sell the company to a bedbank like Hotelbeds. Now hotels are opening every day, things change every day so there's quite bit of people on the ground chasing hotels making deals and getting paid handsomely for it.