Diversification. I sell comics, toys, video games, sports cards, non-sports cards, antiques, statues, misc items...whatever I think I can make money on. If I don't know about a category and am presented with an opportunity, I learn about it and move forward if I can do it. I also price right. I do a search for "lowest price" on the particular item I'm selling and price mine so it comes up first when someone does that same search (which most collectors do). Then I take offers or make offers within a certain percentage of that to move the item. Cash flow is how I run it. I try not to sit on something for long unless I got it for dirt cheap and can afford to sit on it, like that Ali Bop Bag I acquired via auction.
Edit : I just checked my account. I'm down a bit this year but it's sitting at 7.76 items per day so far in 2024.