Sorry if that happened to you. OfferUp can't control when people sell things they shouldn't. It's up to the buyer to fully inspect the item prior to paying for it so you get what you pay for. If the buyer isn't taking the time to be careful and inspect the item as well as doing their homework, it's no one's fault but the buyer when things don't work or end up being something unexpected.
I would encourage you to report the seller that is selling fake stuff! OfferUp would probably ban users for taking advantage because they don't tolerate that kind of thing on here.
You cant expect OfferUp to cover that. Paypal does it for eBay but they just have insurance because money is exchanged through the app for the item so they have some control over it. Not to mention eBay/Paypal is a monster of a company. OfferUp cant do that, they would have fraud all day long. It would be too easy to game the system.
Buying **bleep** sucks but it happens, do your due diligence.