If you are eligible for the "Automatically relist 3 times for free" option you can list a lot more over the course of 3-4 months by using the 30-day Buy It Now option.

Good 'til Cancelled lists an item for exactly one month and then renews it, taking a free listing every time (or charging an insertion fee if there are no free listings left). When a Buy It Now runs out, it should automatically relist itself without using up another listing as long as you are eligible for the automatic relists. I never pay insertion fees but I always have around 100 items listed at any time.

List 40 items in one month, then wait until the 1st of next month (make sure eBay itself has updated so that it says you have used 0/40 listings and not 40 with 0 left!)
List 40 more in the second month (the unsold remains of the previous 40 should all relist once and you will have up to 80 listings active)
List 40 again in the third month (first 40 have relisted twice, second 40 have relisted once, up to 120 listings active)
List 40 in the fourth month (first 40 are on their last trip, second 40 have relisted twice, third 40 have relisted once, up to 160 listings active)