Up-sell - To persuade the customer or guest to buy (or booking) something more expensive.
I.g. -You book a 1-bedroom apartment and when you arrive the host want you to upgrade your accommodation -> Kind of "If you pay a litlle more you can stay in an apartment with 2 bedrooms and balcony."
Cross-sell -> Offering sightseeing packages, pick up and drop off from airport
Up-sell and Cross-sell are different concepts.
As a guest, I would not feel comfortable if a host tried to persuade me to spend more than what I am already spending to book something, being it cheap or expensive.
If a host tried to persuade me to upgrade my accommodation or services that I already had booked, I would not feel comfortable and would inform it to Airbnb or would write it in the review.
To me "UP-SELL" is not acceptable!
As for cross-selling I agree with Laura2592 - ""We don't want anyone to feel like we are hustling them for extra $$ while they stay."
I may change my mind in a future. It is a complicated matter.