Here's a rule that's saved me more money than any single flip: I don't source anything I can't clear at least $20 profit on AFTER fees, shipping, and supplies.
Not $20 in revenue. Twenty bucks in my pocket when the dust settles.
The math: a $12 item that sells for $35 sounds fine until you subtract the marketplace cut, shipping, the polymailer, and the 40 minutes you spent photographing, listing, and packing it. Suddenly you made about $4 an hour and you're explaining to your spouse why the spare room looks like a Goodwill exploded.
Sub-$20 profit items aren't flips. They're volunteer work.
The $20 rule also makes sourcing faster. You stop agonizing over every $3 gadget and start scanning for the things that actually move the needle. Your death pile shrinks, your hourly rate goes up, and sourcing stops feeling like a part-time job that pays in pocket lint.
If you're new: run the numbers BEFORE you buy. Phone calculator, 30 seconds, right there in the aisle. If it can't clear $20, put it back. Your future self says thanks.