For those who don't know Caveat emptor means "buyer beware".
A common thread in /r/travel is "What is the cheapest way to fly to xxx?".
From my experience these discount and third party sites offer only a small percent less that what the main carriers offer. I've never seen extreme discounts on flights.
An inevitable follow up to the posts looking for the cheapest flight is people posting about the difficulties they have cancelling, changing flights, etc. Either they have to pay exorbitant fees to make changes or the place they bought their tickets are next to impossible to get a hold of after they have your money.
And if you try calling the airline they tell you to deal with the company you bought from.
Why do you think "Discount Tickets Here" is so cheap? Part of it is they have next to no customer service and they tack on extra fees for making changes.
Personally, I buy all my tickets through reputable sites like Travelocity, through my AMEX or direct through the airlines.
One time I booked a flight to Australia and after paying I realized I fucked up the dates. I called Travelocity and within 10 minutes everything was resolved with no extra fees.
When I was in Peru I booked flights around the country directly through the airline. When I was to fly out of Lima I arrived at the airport too late. For some dumb reason I read the arrival time as departure. 100% my fault. They put me on the next flight for no extra charge.
In summary, buying tickets through discount sites works fine when there are no problems. But is your vacation worth jeopardizing for the sake of $100?