My parents have done this for probably 5 or 6 years now. They have pulled as much as $15k in a year gross revenue, they usually have 60%+ net profit (not accounting for time). Just a few top-of-mind observations from talking to them about their endeavors from time to time -
-It's harder now than even a few years ago to sell products like this on eBay, etc. Try looking for a specific...anything...on either Amazon or eBay and how much of your page is sponsored SEO-optimized listings that barely have anything to do with what you've actually searched for? My parents are fortunate in that they had a couple years where their eBay storefront was able to accumulate a bunch of 5 star ratings, so theirs is often listed simply because it's a storefront with history and positive reviews.
-Following the above theme, both eBay and Amazon are very, very buyer-friendly. My parents are careful to photograph, document the condition, and describe accurately the products they sell. This takes time to research for certain product categories and even still with how careful they are, if a buyer decides that something is not as described or as they hoped it would be, the sellers are cornered into giving refunds often without any way to request the item be returned. The grand majority of their sales are issue-free, but they spend a disproportionate amount of time on the platform for a small number of issues. These issues must be addressed to the buyer's satisfaction, at the risk of being delisted for a varying length of time. Even with thousands of sales and hundreds of 5 star reviews and a ~4.9/98%+ positive rating, they still have been delisted a couple times for stuff that was absolutely out of their control.
-If the above two points don't concern you, this business is equally about volume as much as it is about selling the occasional 2000% profit item. You'll need to have a good eye for what actually sells and what doesn't, and this in turn means that it helps to do research as you go. Sometimes you learn that the strangest tchotchkes sell like hotcakes, and other times you find that there are things you thought would be surefire sellers don't actually sell at all. Typically when you go to an estate sale (hands down the best places to find items - my parents almost exclusively shop these), you're competing with others who are doing the same thing as you. You have to be able to look at a lamp and know that it has a good chance of selling as much as you need to buy the $5 box of assorted stuff and list it all in the hopes of doubling your profit.
-If it's not clear from the previous point - inventory, and inventory management are key. Maybe 10% of what my parents have listed is actively selling at a given time. There are listing fees for every item, so it's not possible to have everything you have up for sale at all times. They have to take down items and relist them at different times of the year, because you never know when a buyer is seeking what you have. And this in turn means that they have a few tables in the basement with all their inventory carefully arranged.
-I hope you realize by now, that like any business there are winners and losers. My parents have paid $1 for a box of stuff, and end up selling a single item for over $300. But there are just as many sales where they make 10% on the listing. You need to consider the time you spend buying, sorting, cleaning, listing, managing, packing, and shipping stuff.
-$20 per item is unrealistic, no matter if you're selling scrap copper, used furniture, tchotchkes, trading cards, or pretty much any category. You need to come up with the types of things you're going to sell, and then expect to have this be a slow burn of a side hustle.
