Good for you, don't cave to these clowns. You are correct, Offerup does not need your phone number just to let you ask a question to the seller.
It's bad enough they ask for your number, but what's worse is the excuse being used to try and justify such garbage.
I ran into this crap when I wanted to find out some information the seller did not list.
The interesting thing is how no one has complained yet about there being no OPT out option on this. My understanding is that's it's illegal for any company to hold any goods or services hostage in this fashion.
They're literally holding sales hostage under the guise of privacy crap!