You should get your money if it does not arrive.
It should only affect that user when / if they try to make a buyer protection claim.
Seller should have done everything through OfferUp including adding the appropriate shipping label.
Was there any talk about shipping fees before you guys agreed? How much was shipping in the ad?
Ask him for a tracking number, see what he says.
Scammers typically ghost you or deactive their account, doesn't seem to be the case here.
You can also ask support for a second opinion:
https://offerup.com/support/new/