So some of the sellers whom sell through out site have arrangements with either the venue, promoter, or band/team/who and know exactly how many tickets they will be getting and where. Others have access to certain pre-sales and already have their tickets secure. In either case the sellers who can list their tickets on the secondary marketplace, prior to the tickets being on sale at primary, are sellers we have vetted and know can deliver the tickets that they are selling.
Hope this answers you question!
SeatGeek CX Team