Oh, I forgot to mention consigner slots...

My personal recommendation: get rid of the limit entirely. WoW has the auction house, which has no limits on the numbers of items you put up. Given the low sub numbers, this won't have any significant impact, and since it costs money to put the stuff up on the consigner to begin with, it's kind of a dumb double-standard anyway. By the time you get enough people putting stuff up to make it a strain, you will have time to recode the consignment system to be more efficient.