I've only had a quick skim through this thread and so have quite possibly missed someone already saying what I'm about to.
I'm assuming he issued you a Section 21 in this and that you are in England and Wales (Scotland being different).
Was your deposit protected within 30 days of the LL (or their agent) receiving it?
Was the prescribed information presented to you within 30 days of you giving your deposit to the LL (or their agent)?
The prescribed information isn't just a receipt, phone text or single page of paper. It will be several pages long (for an example of what it might look like see here, different schemes will look different though).
If the deposit was not protected, if the deposit was protected late or if the prescribed information wasn't provided or provided late then the Section 21 notice is invalid and the LL has to start the process all over again.
A Section 21 notice isn't in itself an eviction document. It does not oblige you to vacate. It simply entitles the landlord to seek possession via a court / informs you that the LL may do that.
If the notice expires and you are still in the property then you are still the tenant.
Finally, if the sale goes through and you are still in the property and haven't been evicted by a court then the new owners become your new landlord automatically.
Personally I think the LL has been an utter idiot here. They really didn't think about the timing properly. If they lose the sale over this then it is their fault for not understanding lettings and not yours.