My sister in (mostly rural) Wisconsin makes a third income off eBay sales. She also works full-time and rents property in Florida.
Her strategy is to attend flea markets, garage / tag / estate sales, secondhand stores to shop for items of quality, then buy it for pennies on the dollar and offer it on eBay for dollars on the dollar. I questioned the utility of the strategy (since she’s buying on speculation of being able to find a buyer, after all), and she admitted that it’s not 100%, but she claims that the markups on the “good stuff” (and we’re not talking priceless antiques here, just serviceable, quality household items like china, glassware, flatware from the past half-century or so) make up for any losers. And if she misses on an item, then she just unloads it at one of the secondhand stores that she frequents, takes her small loss and moves on to something else.
When we drove through rural Maine last year it was all I could do to keep the car on the road, since she wanted to stop at every “antique store”, garage sale and “alternative lifestyle choice” (people who leave all kinds of stuff in their yard, making you wonder whether it’s a yard sale, a fire or flood recovery, or what).
She’s pretty hard-headed when it comes to investing, business and turning a buck, so I take her word for it that she’s doing well at what she does.