Seller wants to wait a year for tax purposes?

[Butch Greene]

Looked at buying an AirBNB that we know about. House is rented out regularly but also has a 2nd homesite that has a septic installed & perked for 2 Bed. They've got a travel trailer sitting on the septic for the time being.

When we reached out, owner simply said they needed to hold off on selling it until next year b/c they sold their primary residence this year.
Don't know much about tax implications of selling primary residence in the same year as a rental.  

BUT...it's right next door. We could use the RV/septic to build my dad a little house to get him closer to us & rent the other house out to pay for as AirBNB or leased.  

What options do we have for locking in purchase of the property NOW & him avoiding the tax hit of an immediate sale before it goes back on market next year. Don't think a 1st right of refusal would be beneficial.  There are buying frenzies in this area b/c of low inventory.