With pandoc, is it possible to export a bibliography in markdown?

Try using a different markdown format, e.g: pandoc -t markdown_strict --filter=pandoc-citeproc --standalone mybib.md -o mybib-out.md The formats are listed in pandoc's documentation. The default setting is pandoc's markdown which might not generate the bibliography.

Pandoc-citeproc doesn't insert the references in Markdown unless you explicitly specify a div with ID refs:

---
bibliography: 'mybib.bib'
nocite: '@*'
...

Bibliography
============

::: {#refs}
:::

Alternatively, one can also disable the citations extension in the output format, which will cause pandoc to insert the expanded citations:

pandoc --to=markdown-citations …