.dk is Denmark (.nl is Netherlands)
Its one of those mature ccTLDs that can potentially get a lot of type-in traffic in Danish from people in Denmark.
EuroDNS do them for €25, domeneshop.no do them for €15 (or $15)
You should get a confirmation letter from the registry, and you'll then have to confirm online so make sure you use a real postal address (a bit like Nominet for .uk domains).
The only .dk I have bought are for development so no comments on resale. There isn't that much detail on sales on namebio.com.