These days there are many presales - Amex, Mastercard and Fan Clubs. Also sometimes season ticket holders for a venue are given presales so that explains how someone might have tickets before they go on sale. As far as seat numbers. some sellers will list without seat numbers so that venues can not track them and block them in the future. In summary, it is ok to buy these tickets and in a worst case situtation you are backed by the Stubhub fan guarantee.
Enjoy the show and happy hunting.