I have had plenty of unknown limits on accounts that I have made. The limits have already been determined. This is just a glitch within ebay. I think it happens when you are clicking through ebay too fast, but thats just my moronic thought. I have had these turn out to be a 5/500 through a 1000/25000. Sometimes the limits will show up after a couple days. Sometimes they do not. Listing an item will not make the limits appear either. Sometimes an account will never show its limits, so you may just have to test it by quantity with a BIN auction. However, the limits NEVER showing up is pretty rare.
To add to what BJ just stated, sometimes when you get an account that shows no limit, you can list A LOT of items for a day or two and then all of a sudden you get a notice you have reached your selling limits.
My guess is that for whatever reason this rare glitch occurs or maybe those were higher limit accounts to begin with. Who knows for sure right?