I'm in Stockholm and went through Konsumentverket for a similar issue with a different marketplace — not Sedo, not domains, same structural problem. Took eight months but I got the "reactivation fee" waived and a formal apology that I'm sure was template-generated.
The key was proving I never received consideration. In your case nerdbann, the "congratulations" email versus the cancelled transaction is a paper trail. Screenshot everything before they change the dashboard.
Also: