There is no é because you can't have a pronoun be the object of a verbal noun (díol). Instead you do a different structure, using do (or ag in some dialects) and the corresponding possessive pronoun (with proper mutation).
So you get Tá mé do a dhíol (assuming 'it' is a masculine object; so the moon, for instance, wouldn't be a possible 'it'). However, in the Caighdeán, do a becomes á. So you get Tá mé á dhíol.
And no, 'them' would not be accepted - that'd be Tá mé á ndíol. And Tá mé á díol would be if you were selling a feminine object, such as the moon (an ghealach)
I have to ask again. Did I understand it right if the object of a verbal noun is a pronoun I use "do+á+lenited/eclipsed form of the VN". If the object is a noun I use Bí+subject+ag VN+object (in the genitive case)? So, I am selling my car, would be: Táim ag díol mo chairr. But I am selling it, would be Táim á dhíol.
It looks like you've slightly mixed up the active and passive progressive structures. This is pretty tricky, and it doesn't help that Duolingo has many exercises where the English active structure is translated with the Irish passive one. I've given a couple different sentences that hopefully help elucidate the difference.
Táim á dhíol - "I'm selling him/it"
Tá sé á dhíol agam - "He/it is being sold by me"
Táthar do mo dhíol - "I am being sold"
None of these quite mean "I am for sale" which I'd tentatively translate as Táim ar díol. However I think it's unlikely the English idiom directly translates into Irish i.e. I'd wager Táim ar díol is much more likely to have been said by a slave on the market in Viking-age Dublin than an easily-bribed Gaelic politician