An bhfuil sé á dhíol? can be either "Is he selling it?" (active voice) or "Is it being sold?" (passive voice).
The saorbhriathar in Irish (An ndíoltar in Éirinn é?) is usually translated as a passive voice sentence in English ("Is it sold in Ireland?") but it is actually in the active voice in Irish.
An bhfuil tú a dhíol? doesn't make any sense - "Are you his selling?". Even if it did make sense, you're equating a pronoun "you" with the noun "selling", so you would have to use the copula, not an bhfuil.
Change the "it" to something specific.
Tá sé ag díol an tí - "He is selling the house"
Note that it's the genitive an tí after the progressive ag díol. But that presents a problem for a pronoun, and instead of ag díol é, you use á dhíol.