"Are they selling it anywhere?"
Translation:Prodávají to někde?
It is correct a correct Czech sentence. Possibly a slightly different meaning than the default translation. I am not that good in English to know all the nuances of the English original, but it might cover both.
Your word order would be used, for example, when you were surprised, so you repeat the previous declaration with a question.
You've answered yourself. "Prodávají to jinde?" means "Do they sell it somewhere else?" It's correct, but it's a different sentence with a different meaning.
English changes "somewhere" into "anywhere" in questions (and in negative statements), but it's still the same "somewhere", i.e. "někde" in Czech.