Buying plane tickets from Webjet, Cheapoair, etc.

To be doubly sure, phone SAS and ask. If the reply is "no flight" then you know SAS won't be flying you. Some of the on-line agencies also have service representatives answering their phones so that would be my next call in search of an explanation.
The general opinion on forums such as this one is that on-line agencies may not provide much help if (when) something goes wrong. In my one experience with a problem reservation, I received a telephone response from Cheapoair to my e-mailed complaint and eventually a refund appeared on my credit card statement (although I also complained to the card company so I can't know which one achieved the desired outcome.)

We and our daughter have used cheapoair, travelocity, orbiz and expedia without problems.
But last month I booked through faredepot.com, and after the fact found that they had given us a connecting flight that was not one of the listed combinations .Lufthansa, EWR->DUS-TXL, I had clicked on the connection being the7:10 AM flight, after landing at 6:10 AM, having debated whether I could make that or needed the 8:40 connection. The payment page showed my selection, so i did not examine the confirmation e-mails carefully, since I saw the starting and ending points. Two nights alter I found when I went to give our rental our arrival time that we were on the 10:15-11`:25 connector to Tegel. Fascinating, since faredepot was not even offering this. Of course I got nowhere with them, and did not have a screen shot of the selection and payment screens (why would I?) to send to my bank. Not a killer, but that's 3 hours of additional waiting around jet lagged at Dusseldorf when we could be out walking Berlin.
With these smaller various 800 services, which i wonder if they are not all the same place (not Orbitz and other biggies I mentioned above) I have noticed other bait and switch, including "you have to call for this deal", select to find out the details, and upon selection finding out "this is no longer available".
Yes, these are legitimate in that actual tickets which you can find on the airlines booking page will be issued. But I do not think they are worth dealing with unless you have to save every last dollar (in which case, can you afford the trip?)

Cheapoair is the worst company I had to deal with ...
The worst service - It seems that they don't know the signification of the word "service" and customer.

It's not a problem to use these sites if you know what you're buying, you're happy with it, and there are no problems. These places sometimes piece mill together flights from various airlines into one complete package...which is fine if the airlines cooperate with each other, but a disaster if they don't. We bought a flight from DC->Moscow / St Petersburg -> DC. The flight to Moscow was routed through New York and we found ourselves having to claim our luggage in NY and drag it to a different terminal OUTSIDE of security which meant we had to go through the entire checkin process again. Luckily we had lots of extra time so we could physically do this. Coming home from St Petersburg we connected in Moscow and AGAIN...found ourselves forced to claim luggage and go through the whole checkin process in Moscow. It was a lot of work and we were exhausted, but we managed to make our cheap tickets work for us.

Just know what you're potentially getting yourselves into. And if you happen to have a problem don't even THINK about contacting the airlines for help. They're already rude and unhelpful enough, but as soon as they see you didn't purchase that ticket directly from them all communication ceases and you'll be told to figure it out yourself.

I had a problem once with a third-party ticket seller. Never again.

They are legitimate sites. I don't quite understand why there are sometimes big price differences- they probably get deals for buying bulk seats from the airlines.

Even though they are legitimate-if something happens to your flight, you are on your own. If your flight gets cancelled because of weather, or you are late and miss a connection, etc, they may or may not actually help you. And the help frequently involves a fee. And the airline won't help you if you didn't buy the tickets directly from them.

I only purchase directly from the airlines. If you see a big difference in price compared to a third party site, try calling the airline. They may be willing to give you the lower price. Or the posted price may not yet include fees, airport taxes, etc