The problem with bazaars is finding who is selling the things you need. A bazaar is tied to a character, and a character can be anywhere in the world. Even if you find one selling something you want, there's no easy way to compare prices on what that character is selling.

Unless these problems are solved, please, PLEASE no bazaars. FFXI had a bazaar system, and lots of items that could ONLY be sold on bazaar, not on the general auction house. The only way you could effectively SELL under this system is to park your AFK character in one of the unofficial bazaar selling areas (which at its worst got so congested SE had to ban bazaars within certain areas for a time) for hours or days at a time. The only way you could effectivley BUY under this system was to spend a half hour or more examining bazaar after bazaar, often ending up never finding what you're looking for at all (or finding it vastly overpriced).

However... if these problems ARE solved, what, exactly, makes a bazaar any different from a marketboard?

I honestly have a hard time understanding why anyone would be enthusiastic about a bazaar system. The marketboards work wonderfully well just as they are.