The "Whom...to?" version is also an accepted translation, but that extra "to" is required, either at the beginning or at the end. But I hadn't noticed the switched "you are", which must be in the other order since this is a question.
That exact sentence, was, indeed, missing. We had "To whom are you going to sell the gemstones," and we had, "Whom will you sell the gemstones to," but we were missing the exact combination of "whom...to" with "going to". It's in there now.