Each subsection (Battle, Spell, Mender, etc) needs to be capped and/or require retainers to be at a stall instead of dawdling and cluttering up the floor. Would need to enlarge the number of stalls by 2-3 times, but I think it would funnel players to the other wards once Ul'dah fills up.

I usually drop my wares at Gridania just out of principle. Surprisingly, I haven't had any problem selling stuff.