Just because it has a codeshare number doesn't mean it'll work with a DL fare. A lot depends on the origin/destination and routing being flown. Ie It may work when flying JFK-AMS, but not ATL-AMS due to married segments.

DL van sell a KLM fare, but generally that'll mean calling in. Of course the KLM fare, even though on the exact same flights may be more, the same, or less than the DL fare.