Some searching has lead me to believe that this bug has been reported before but it has always been dismissed - someone responds by pointing out that you cannot build a world achievement if another player has already started building it.

There does appear to be a separate bug that affects the Capitar faction (possibly others, but I've never been able to build the merchantcross bazaar as capitar after several tries, whereas I often can as other factions).

Before you say, 'the plural of anecdote is data', to demonstrate the error state I started a new game on novice difficulty, with only a single opponent: Lady Irane. I should have no trouble beating her to Trading, since she will have more unrest than me and I start with Civics.

On reaching Trading, I immediately looked at the build list and sure enough, no Merchantcross Bazaar.

Here is a pastebin of the debug.err log for my game (I've cut out the irrelevant parts):

http://pastebin.com/JLHMjMfp

As you can see, on Turn 2 Lady Irane begins researching Training - she's not even going to be close to the prereqs before me. I get to Trading on turn 10 (kept my ruler in the capital for unrest, enchanted for growth, and went conclave asap, while beelining Trading).

There is no indication in the log that Lady Irane changes research or does much of anything other than acquiring Othollo.

And yet, no merchantcross bazaar.

Here's a link to the save game I made after reaching Trading:

https://dl.dropboxusercontent.com/u/2174227/no%20bazaar.EleSav