It looks like they buy and sell from other sources and include TTM (through the mail) autographs on their web site as well, so their authenticity will be limited by their own personal expertise.
Many collectors these days, especially of modern celebrity and modern music, prefer to buy from sellers who obtain all of their own autographs in-person themselves.
You can reference this list for many good examples of top in-person autograph seekers: http://www.racctrusted.com