One word of warning (that doesn’t concern the console itself as much as the entire process of shopping) is that it’s much better if you buy your console in person. Of course, if you’re buying from the previous owner, make sure to meet at a public place and do not pay them before receiving and inspecting the product.
Buying the console in a face-to-face trade gives you an edge — you can make sure that it has no visible faults and that it runs well. This is harder to do online, but still achievable. If you found an online listing that includes shipping, ask the seller for very clear photos and maybe even videos of the console before you commit to a purchase. It’s also advisable to use trusted platforms, such as eBay, instead of sending money to someone without any buyer’s protection policies in place. Ideally, the platform you shop on should support you in the event that you have to return the console because it wasn’t exactly as advertised.
Another tip is that you should try to get the original receipt from the seller, if applicable. This will be needed if you need to use the warranty, assuming it is covered by one. Of course, this doesn’t apply to older consoles, but if the seller bought it relatively recently, it’s a good idea.