I've used Ebay to sell my old items and made some money back, and very occasionally I'll spot a certain item in a charity shop and re-sell it for a profit.
I once bought a huge bundle of niche items from eBay and sold them individually over 12 months for £500 profit.
So you can make money, but it's HARD work. Buyers can be (at best) idiots, or (at worst) scammers.
After listing fees, completion fees, PayPal fees, postage costs...well, for example, if you buy something for £2 and sell it for £10, you have a profit of £8. But your eBay fee is £1 and PayPal fees £0.50, postage £2.90, packaging £0.50 then your profit is reduced to £3.10. if you've had to drive to the post office, you'll need to deduct petrol.
So...with the half hour it took you to list the item, photograph, describe it and the time taken to go to the post office, you're working at £3ish an hour.
You'll need to be making bigger profits and selling high numbers stock to make anywhere near minimum wage hour after hour, day after day.
I do it to make a few quid on the side, but I'd never consider it as a main source of income.