Copenhagen Train (Central vs. Airport) to Stockholm

Avoid Rail Ninja - as you found out. Reseller/agency, not reliable.

I found some trains (random date) on the official sj.se Swedish website. Surprisingly, I couldn't get any trains on the official Danish DSB.dk site. I have used them in the past with no problems. Maybe I did something wrong. I do not know how early the Danish and Swedish train operators release their schedules and tickets - possible it is still a bit early. Sometimes a month in advance is it for some European routes.

Some use Trainline.com regularly with no problems. I have not personally.

Check also the advice from Man inSeat61. Generally very reliable advice and good info. Scroll down here for some ideas and other booking site suggestions Copenhagen to Stockholm: https://www.seat61.com/international-trains/trains-from-Copenhagen.htm

1) No. Rail Ninja is a "travel agent" based in Hong Kong whose business model is scamming people by selling them extremely overpriced train tickets that might (or might not as you found out) be genuine.

2) Buy it direct from the company that actually runs the trains, www.sj.se. Just keep in mind that Copenhagen is the English name. In Swedish the city is called Köpenhamn. You can also check what Snälltåget offers, they have two daily departures from Copenhagen to Stockholm, https://www.snalltaget.se. Their website uses the Danish spelling of the city, København.

Surprisingly, I couldn't get any trains on the official Danish DSB.dk
site. I have used them in the past with no problems. Maybe I did
something wrong.

Not at all surprising, SJ operates the trains. DSB doesn't operate the trains.

@badger & @ordtraveler - Thanks for the info and tips. Much appreciated! I will check those out and make sure I use the correct domestic spelling.

-SW