Did a quick search and didn't see anything like what i'm about to propose, but it's not like i read through every thread. So don't flame me please.
Why not allow only one retainer per player to be summoned for each city's market wards? It wouldn't break anyone's bank because you can still sell your "high end" stuff or whatever in Ul'dah, but it would free up room in Ul'dah's wards for other people. I'm willing to bet at least half of the retainers in Ul'dah are players' 2nd. It would also force people to set up their second retainer in another city, if they so wish to, which in turn would get more items listed for sale in the other cities.
And while i'm on it, why not allow the text search to find +1 items? That's really all i want to use it for but it doesn't seem to work. The market is so flooded with cheap NQ mats that HQ things rarely show up in the price list. Spreading out retainers to the other cities would help alleviate this problem as well i think.