My eBay profit margin has been roughly 30% for the past four years - down from 40-60% for my first 18 years. There are several reasons for that. Like the OP I now sell primarily items in the $10-$20 range plus shipping - selling higher priced items has become too risky.
All the shipping increases in recent years have resulted in my lowering the price of the item as well as paying more in fees calculated on shipping cost and then sales tax. Cost of goods sold has not changed much because most of what I sell was purchased years ago when I also had a brick and mortar store. I rarely go to thrift stores or yard sales any more. Since the pandemic thrift stores have become too pricy and the "good stuff" is often sold online or before it hits the shelf. Estate sales prices are even worse.
On the positive side the federal government is getting a smaller take of my profits.
