neon, you are confounding three very different animals.
Seatgeek is an aggregator of secondary market ticket sites. This mostly consists of brokers or disguised brokers. They do no direct sell.
Stubhub is a marketplace. The sell tickets from anyone who lists there, which could be a principal, an individual or a broker/disguised broker.
Only Broadway.com is a broker.
It is ridiculous to tell someone to only use telecharge/TM, because (a) they may have no seats and (b) the price may be cheaper on stubhub or on a site linked through seatgeek. Avoiding Broadway.com is invariably the right advice.
At LCT there are often seats available near the performance date. This is because of both house seats and member seat asides. Some people may not want to wait, because they need to lock in seats within a narrow time frame. This may lead to overpaying. There are also people who want quality seats and may find that what the box office has are inferior locations. It all comes down to what someone wants/needs, and what they are willing to pay.