In theory this may sound great, but it is unrealistic because used dealers will always find a workaround. CL has tried different methods over the many decades of existence. Even having a filter does not help, since individual sales people from used dealers will post the ads with their own personal accounts.
The only real method to ensure the distinction between vehicles being sold from a dealer and a private individual is for a visual confirmation of each and every title, so that the name on the title matches the legal name of the OfferUp seller (private seller). Otherwise, everything else would be considered a dealer/cubstoner sale.
If you want to block all the dealers, you will have to block them individually as there is no filter available on OfferUp.