Unfortunately a class action lawsuit isn't really an option it actually says some the terms of service that you agreed to that you can't Sue offer up.
Actually a class action lawsuit can get certified, IF OfferUp violates its own TOS, then the arbitration clause becomes invalid.
Case in point: All of those that have shipping damage claims against UPS are unable to file their claims with the carrier because OfferUp refuses to indicate their client/customer number (OfferUp is a client of UPS), which UPS has specificed as a REQUIREMENT for the claims to be filed.
The irony is that OfferUp has replied to these users to deal with UPS directly, but will not relay the REQUIRED information for the users to file the claims. This is an example of some classic "Catch-22" FUBAR.