Scams when selling on Craigslist or ebay

Have a couple friends get C/L scams pulled on them.
The 1st, she was sent checks that were bad. She even showed them to me and I told her it was scam. I asked her "why would someone send you a $1500 check for a $50 purse?". I thought she understood. She took the check and cashed it. It bounced into orbit about a week later.
She got a second check a few days later, this time she called the police who told her it was a scam.
The 2nd scam was scarier. A friend found a truck on C/L for an incredible price. Called and was told it was part of a divorce and had to be sold quickly. He left with $8000.00 cash to meet the guy. What he met was a couple of thugs, with guns, who knew he had $8K on him. Easy pickings.
If it looks to be good to be true, it is.