If you're selling on eBay, you can set up
buyer requirements that limit who can bid on what you're selling (you can also set up exemptions for specific buyers, and block specific buyers).
For larger-ticket eBay sales ($500USD or more), you can use the recommended escrow service at
www.escrow.com. You need to understand how this works. There's an inspection period and the funds are automatically released at the end of the period. The buyer has to reject and return the material if it's not satisfactory, and you then have five days to examine the returned goods. If there's a disagreement, there's a dispute resolution process. No money is returned from escrow to the buyer until you as seller agree the transaction is complete or terminated. There are cases of
escrow account fraud on eBay, so beware of a buyer suggesting an escrow account other than
www.escrow.com (the escrow account eBay recommends).
If you're buying on Craigs List, you're on your own. If I were selling a big-ticket item there, I'd set up an escrow account for payment for protection and for low-ticket items, I'd only deal with people in my area so I could meet them face-to-face.