What happens if I have buyers and I hit the limit for the month?

Am I just left out to dry?

Yup, pretty much. I had the same thing happen a couple of weeks ago. You can contact eBay and request an increase.

So how can I prevent this? Do I have to manually take down all my listings, even if they are doing well, right before I hit 100 sells every month (this is assuming I finally get past the 90 day limit) or 30 sells so I don't have a bunch of angry customers with no product?