Don't get lured by prices that seem too good to be true - that's usually the first sign something's off. Beyond the ratings everyone mentions, actually read through the negative reviews instead of just scanning the star count, because you'll spot patterns like "never shipped" or "item description was completely wrong" that might not show up in the overall score.
The big thing people miss is checking whether the seller will agree to use the platform's built-in payment protection rather than asking you to pay outside the app or via wire transfer. If they're pushing you toward payment methods the marketplace can't dispute, that's a massive red flag. Also, for something as expensive as a laptop, try to meet in person if possible - you get to inspect it, test it, and see the seller face-to-face, which cuts your risk way down.