If you're looking for a used Mac, the best source is the Apple online refurbished store.
Yes. Too many people have posted here about losing all of their money with ebay scams.
Not sure if I'd buy a Mac on eBAY but, to my mind, buying at the refurb store pretty much defeats the purpose of buying used, since the prices are so close to new as to not make much of a difference. That's not to say the refurb store isn't a good deal for those who want an essentially indistiguishable-from-new product at a modest discount, since that's what the refurb store offers.
But if you're buying used to get a serious discount, your best bet is probably to buy locally on Craig's List (if you're in the US), or something equivalent. Of the 5 Macs I've had, four were new and one was bought used on Craig's List (for about half of what it was going for on the refurb store, even though it was in mint condition and came with AC+), and they've all been great. The way to do it is to tell the seller you're serious, negotiate a price on the phone (so the seller knows they're not wasting their time), and tell them you'll need about 20 minutes to do an inspection and run some tests (you'll want to download the aftermarket apps they mention onto a USB stick):
There's plenty of life left in second-hand computers, especially if you happen to be shopping for an affordable student computer before the school year begins. With many districts implementing remote learning, the student computer is more important than ever, but how do you know if what you're...
appleinsider.com
In fact, to be completely up-front (and test the seller to see if they'd balk), I emailed the seller that article beforehand so they'd know the tests I'd be running before I got there. If they're someone you want to deal with, they'll have no problem. If they give you a hard time, look elsewhere. Ideally, if it's a late-model Mac, you'll want to get one with AC+ so you're protected against any unseen defects.