When it's something expensive and easily steal-able, I make a token effort to ensure it's not stolen. That means either:
1. Meeting at the seller's place.
2. Seeing a receipt of original sale or some other evidence of ownership.
3. Seeing an ID so, if it's stolen, I'll know who sold it to me.
4. Checking against registries for stolen things. Not all stolen things are registered as stolen, but the thief isn't willing to take the risk.
Refusing #1 and lacking #2 is completely understandable, but if they also refuse #3 and #4, I'd be really suspicious. Fortunately, I've never had a seller do that.
Oh, and when it comes to expensive stuff, I really don't feel comfortable counting money in a coffee shop. It quickly attracts attention. It would be quite plausible if someone noticed the money, followed the seller, and robbed them (or just stole their bag).
Now I wonder why the idea didn't occur to me earlier: Meet at a bank. It's safe, secure, and it might even be possible to avoid using cash. I'll definitely consider it the next time I sell something on Craigslist.