The reason I ask is that my friends and I are trying to book flights to Mexico during the christmas leave period and if we go to the prices shown directly on the airline, then we are looking at about prices of 4k.
However when using websites such as skyskanner etc they have prices for 2.8k for same travel periods.
How is this possible? Are they selling us tickets that don't exist and there is a high risk that those tickets will get cancelled?
I have used online travel agencies for stuff like accommodation no problem but never flights and a lot of these have terrible reviews online
Expedia and their network of sites (like Hotels.com) is generally reliable and fine to book. You're unlikely to get cancellation, you can input the 6-digit number in the confirmation e-mail into the airline's website directly to confirm the flights.
HOWEVER, those travel agencies are able to offer cheaper rates because of restrictions placed on their ticket offerings. Especially for things like changes or cancellations which are either quite pricey to do or not allowed at all. This is where majority of the bad reviews come from, people getting stung because they agreed to something they didn't read.
Make SURE you read all the T&Cs and understand what you are buying. Otherwise, all good.
Not all travel agencies are reputable, make sure it's one of the big chains and not some random one. IATA registration is a good first pass filter.