I tend to think it depends on what you compare it with.
B&M stores have much higher costs.
There you would calculate your total revenue per square feet. Your eBay business is just a mouseclick away.
Even a yard/garage sale can be more expensive considering the time you spend in order to drive a certain amount of traffic. Try putting your stuff out in the yard for 7days ...
You pay for the traffic and the exposure on one of the biggest marketplaces in the world.
In the moment you spend your money on additional listing gimmicks like "bold" "framed" and such you might want to reconsider because that's where its getting ridiculously expensive.
________________________________________________________________________________
The easiest way to make money on eBay? You should have bought their stocks years ago.
Now it's just hard work.