My best shots have been small lots, more than most people want, less than merchant lots. and Buy it Now.
I think I lose money on ebay, I may profit on one item, then buy something I need. While you do have to collect and pay sales taxes within your state, the whole question of income is vauge.
Transactions amounting to a few hundred a month, both buying and selling do not reach reporting levels.
I like ebay for camera, computer, motorcycle, and microscope parts. I might pay 10% of dealer prices. It is the big junk store. I am more of a buyer. I do sell enough to pay for internet, toner, photopaper, fusers, memory.
The best thing to sell is other people's stuff. You are in the right place for machine tools, a few pictures, a good description, post it, with a reserve that covers cost, and if it sells, the seller is happy, and if it sells for more than the reserve, you are happy. It is a no investment way of picking up thousands.
Marketing works, good pictures and description bring good prices.