How can you make sure you don't just end up spending money with no profit when trying to flip items?

Look, when it comes to business, or investing, there is no risk free option. Also, most of the time, there's an inverse relationship between risk and reward. You can, of course, stack the odds in your favor by being knowledgeable about your products and your market, and about business in general, but if there was some kind of guaranteed money maker, everybody would already be doing it (and then the market would get saturated, and it would no longer be a guaranteed money maker). Now, the closest thing I can think of to what you're asking, would also mean probably making relatively little money, with relative little consistency, and that would be to only flip items that you managed to obtain for well below what they're selling for used, right now. Like, if somebody offered to sell you a corvette for half of what Carmax would buy it for...then of course. But those deals are few and far between, and if you knew how to find them consistently, you wouldn't be on here asking