In several posts you comment on the negative impact that self-publishing may have on securing a traditional publishing contract for a novel. Things like poor sales record online, can’t be pitched as a “debut novel”, etc.
In your opinion is there any downside to republishing a previously published short story from a professional print anthology, if rights have reverted to the author? There seems to be a demand for short fiction on Amazon and I wondered if having previously published short fiction online might potentially have the same negative impact when pitching a debut novel? What if it is a novel based on the short story? I have not gone that route, but if I was to do so, would there be an expectation to mention that in a query?
Republishing short stories is fine. Stories are very different from novels in that respect. For starters, you aren't really responsible if it only sells one copy. Editors expect stories to sell in low numbers.
If the novel is based on the story, you're still ok.
If you've had a short story selected for publicaiton, whether or not the novel is based on it, you mention that in the publication credits paragraph of your query.