The author needs to login to PSN or XBOX live, and create a Rockstar Social account. That unlocks the BAWSAQ market which is based on other players trades.
I don't have a Rockstar Social account and I've managed to use the BAWSAQ market, it opens during some of Franklin's assassination side missions given by Lester. It's not under maintenance throughout the entire game.
Hmm, I'd have thought it would be tied into GTAV online multiplayer, which doesn't open until tomorrow. In any event, market PvP might provide an interesting counterpoint to actual PvP in the multiplayer environment.
(My fiancee's gaming clan is planning to be in GTAV in a big way, starting tomorrow.)