There was once a very reliable way to forecast eBay sales. If you had 100 listings for 10,000, and increased your listings to 200 for 20,000 - your net numbers would double. As you grew, your sales grew. That stopped working a couple years back, as many more LEGO sellers entered the market.
Nowadays, unless you can keep growing, your sales will decline. As bad as that is, hopefully it will stop the influx of new sellers (since it is less worthwhile) and the market will stabilize.