One of these sites offered the ticket for dozens of Euros cheaper, and allowed me to go through the booking process until the end: I entered my CC, hit "Buy", and the next page was "Oops, the flight is no longer available at this price. The price now is: [the same price as booking with the airline]. Would you like to complete the purchase with this new price anyway?".
Fucking hell, it pissed me off so much that I said no and booked it with the airline after all.
I'd recommend never ever using reseller agents. They frequently mess up tickets, they sell tickets that don't exist at all, and most of them won't forward your contact info to the airline's system, so if there's a problem or delay you'll never receive the notification from the airline. (Including coupons, hotel room offers in case of cancellation and other stuff)
The airlines may be bad but the resellers are worse.
So I'll typically use a service like Google Flights to find a flight, pick an itinerary, but then go to the airline's web page to book the flight directly.
Same for hotels. Find a hotel on Expedia, then book with the hotel directly.
I've seen too many horror stories of people booking through a reseller, and if something goes wrong, the reseller tells you to talk to the hotel/airline while the hotel/airline tells you to talk to the reseller.
Save your points and use them on hotels instead, where the experience is just as risky, but at least fails in a less spectacular way when it goes wrong e.g. unless you're booking with an agent in a high demand area where there are NO hotels you usually can leverage a backup plan a lot easier than if you are stuck in an airport.
I've never been stranded when I've contacted the hotel directly, via their website or the phone. Sometimes a glitch does happen, but when it does the hotel will call around on your behalf and find you a room.