If you are referring to..."sellers must enroll for the service through Stripe. Enrollment in and usage of the OfferUp Payment Solution is voluntary, so buyers should note that some sellers may not accept Electronic Payment Methods, or may only accept Electronic Payment Methods for certain transactions."
Do you see the word "VOLUNTARY" within the 2nd sentence? The OfferUp Payment solution is an ONLY an OPTION, but a requirement IF you want to SHIP through OfferUp.
The "must" that you are referring to is IF the buyer wants to utilize an alternate form of payment (OfferUp Payment Solution), the seller MUST be enrolled it in as well.