Check serial number before buying

Apple doesn't keep records of stolen Macs.

You should make sure the Mac was prepared for sale according to this Apple document: What to do before you sell, give away, or trade in your Mac - Apple Support. If it isn't prepared that way stay away from it.

Consider getting a refurbished Mac from Apple or OWC (MacSales.com). Those are the only two sources I'd purchase a used Mac from.

NEVER EVER buy a used from from a source such as E-Bay, Marketplace, Craigslist, your local newspaper. ONLY purchase a used Mac from a reputable source such as Macsales.com or buy an Apple refurbished Mac directly from Apple. When you buy used these more often than not tend to be nothing but trouble, you have no idea what you are buying and in most situations the seller has not properly prepared the Mac for resale which means you simply inherit a huge mess!