I used eBay so I could get a refund if the laptop wasn't as advertised. I spent weeks looking at new listings looking for a good deal. I eventually found an amazing deal from a hospice that was selling excess stock. I've worked in a hospice before and know this would have only ever been used sparingly in an office and be very well looked after.

On eBay I would avoid anyone who hasn't written out a complete description and detailed pictures of condition and specifics. Like the other comment says, the BIOS being unlocked is very important. Read descriptions carefully. People fall victim to buying expensive things that can't be returned because it was mentioned in the listing (e.g. buying a box only for a very expensive price). For any laptop I find, I search for forum posts from other users about how that model works with Linux and videos for a teardown to make sure that RAM, WiFi module, etc can be upgraded. Make sure the charger is included.

Search eBay for "8th Gen 13 inch 16GB", then sort by lowest price for buy-it-now. That's what I did for a number of weeks. Got one for myself and a great one for my dad as well. Good experience both times.

I wouldn’t buy a used MacBook from an individual seller unless I could meet in person to verify there’s no BIOS/TPM lock going on that would prevent me from doing a secure erase and wiping the SSD to start fresh. A laptop with a replaceable ssd is probably less of an issue, but I’d still feel more comfortable having a picture of the BIOS showing no password set or anything, and a picture of it booted to desktop at minimum so you know it isn’t a stolen laptop that has a password no one knows. If you’re buying from like a second hand recycler or something, anyone that sells through significant volume of devices, I’d be much more comfortable just pulling the trigger sight unseen.