I think the toughest thing so far was figuring out shipping. Here you can buy the shipping labels through your eBay store, and it saves you a few bucks on each one... We also have a different rate for media mail, so shipping books/games/etc is discounted. Getting that shipping cost as low as possible is a must.
I personally like to photograph items, box them, weigh/ measure, and then label the box with a detailed description. I have a box mountain, and this helps me stay organized. If you include part of the listings title on the box, it saves sanity haha! This way, you also know exactly what the shipping cost is, and you can decide how you want to approach covering it.
Other than that... Sell through ratio is important to note- you can go to the filters and select "sold/completed" and see how many of an item has sold vs how many are listed. That's the best way to know if that cool item you found in the thrift will sell or not. Also, you can write everything off on taxes here, guessing it's the same there? The shipping fees, packing tape, the square footage of any room that is used to photograph/store items...
Hmm, what else do I have... Oh! Save all of your son's clothes that are still in great condition after he outgrows them, you can totally get back some money for them all! That's one of the main things I am selling. I make lots by size and try to keep them around 20 items each for less fancy clothes, and smaller lots for dress outfits etc.
I thrifted most of my kid's clothes, so I'm getting like 25-50% back on what I spent on them, not bad at all! Hope some of that is helpful, it really is a nice side hustle! 😄