Quoted from YeOldPinPlayer:Specify in your ad all responses must include the item they are inquiring about and a time they are available to pick it up.
That weeds out the vast majority of scammers. Many scammers are auto-responding to every ad. They spend no time reading or complying with specifics.
OfferUp has been notorious for scammers sending immediately a response and another giveaway that they are scammers is that they don't have history in their profile and want it right away.