from my observations, it depends more (if not only) on the number of people with an item on the market... not on the total number of items that are posted. for example, oreputty and diamonds may both be able to handle 100 characters' items, but since the average person would have 70 oreputties and only 50 diamond nails, there would be your difference. if all 100 slots are full at 50%, you can't post anymore, whereas if 99 slots are posted at 50%, and one at 75%, you could post at 50, bumping the high one. then it would be full at 50% til all of someone's items are bought.
now, i'm pretty sure that not all items have the same number of "slots", but i haven't really checked that out. not sure how i would, unless i had 50 or 100 characters to try it out with.
--jason