@David993,
if I understand you correctly, your host assured you that your stay will not be affected by the sale of the house. Sales usually take some time and if he had no bites yet on his offer, he may be quite sure of it.
He may also be an honest person and intend to hand over the place not before a given date anyway, because of you, because of him needing to find a place for his furniture first or for many other reasons.
Nevertheless , you wish to cancel for free.
It seems there is nothing your host could do or say to reassure you: you wish to cancel.
Your suggestions to shoft the risk of an unforeseen event to hosts are pretty far reaching:
you paid with the booking, but the host gets paid only after your arrival anyway. To make matters even stricter, you suggest, that every host, for every booking, deposits 50% of the rent, maybe for months ahead? Seems an absurd notion to me. Most hosts won't have that kind of money, could not finance half their turnover interest free for weeks or months and would simply leave the platform.
If you are so worried, you should book classic hotel stays. Airbnb is a way to share, people share their homes and the prices are lower. The risk is shared as well. Not equally though, it's already vry much in favour of guests.