I recognize that there's a certain strategic weakness to sharing these precautions. Assuming, at least, that craigslist-oriented predators are googling the subject. Who knows.
In the end, you can't absolutely forestall danger, but you can be careful. Here are some things I recommend.
First of all, if you can go with someone (even if it's two women), you are much safer. That would be the simplest precaution.
Since I generally don't have that option, I do a number of other things. I always get the seller's address in writing. Then I send it (along with his or her name, phone number, and email address) to someone who immediately answers the phone at the time I'm leaving. (I note that this is rarely my husband. If he's not answering his phone, he's not the person I need for this purpose.) I tell that person what time I'll arrive at the seller's home and what time I will be giving a call to say that I've left and I'm fine. If the person doesn't hear from me, he is to direct the police to the location I've indicated. And I make sure to call afterward. I casually let the seller know (often before I show up) that someone else knows exactly where I am.
In fact, I almost always tell the seller that either I, or my husband, or both, will show up to pick up the item - but I don't yet know who. (Sometimes this is true. Sometimes it isn't.) The number of serial killers who are looking for either a man or a woman or both has to be small, right? If the person is unconcerned about who shows up, he's probably on the level.
I also follow basic safety precautions - I don't ever let a stranger follow me into a space. If the item is in the garage, I will walk after him (and of course I'm looking to make sure there's no one else behind me). Ideally, I want to see signs that he brought it out so it will be more accessible to me, and safer for me to take a look at it (this is what any person selling an item would naturally do anyway). Exceptions for people with clearly limited strength, who are less threatening anyway. If I am alone, I will not follow someone into a basement, alley, warehouse (unless there are other people around), or second story. Better I should not have the item.
If the item is really easily portable (a guitar, for example), I don't even offer to go to the person's home - I suggest we meet at a local McDonald's (any other populated area would also work). No sensible person would refuse - this way, he doesn't have to give me his address.
While I do provide my phone number, I never, ever give my address (if I'm buying). For this reason, I never ask anyone to deliver anything. This seems obvious, but sellers will sometimes ask you where you live in an effort to offer directions or negotiate a helpful meeting spot. In that case, I always say that my GPS will get me to the correct address. I am also willing to say the state (remember, in DC that makes a difference) or county where I live. Obviously my street address is not an option. If someone asks a question I'm unwilling to answer (or offers to give directions via phone rather than sending a street address), I simply explain that I'm sure they're all very nice people, but they are strangers, and safety forbids me to proceed in that way.
If I'm going into an area I don't know, I look at crime reports and aerial photography (and check out nearby homes and businesses) before deciding whether it's worth it. And I have an absolute ban on buying items in Baltimore. There are, of course, nice neighborhoods in Baltimore, but I don't know the city well, and there are far too many dangerous neighborhoods for me to chance it. This rule eliminates the best architectural salvage (and prices) in 100 miles, which is a big sacrifice, but it is necessary. I made one exception (for my kitchen cabinet) after verifying with aerial shots that the neighborhood was really nice. When I showed up, it was a slum (with large lots and swimming pools - explain that, if you can). I got a beautiful cabinet and the seller was very nice, but I will never make that mistake again. It simply is not worth the risk.
I also think carefully about whether the ad is too good to be true. Of course, as we've discussed, there are some super bargains on craigslist. But if something is a high-demand item and a crazy price, and after a week it's still available, there has to be a logical reason. If logic tells you it couldn't work out the way it has, stay away. Similarly, I ponder whether the item I'm buying would be the bait a psycho would use to reel in someone like me. If you're looking for a young woman alone, you do not advertise a six-foot solid-oak armoire. (Although I did indeed show up alone for that one. And met a delightful family of four.) I worry that much less when the thing I'm buying would be lousy bait.
In the end, of course, you have to go with your gut. Take in all the little cues that aren't directly relevant to your purchase. Does the person selling baby furniture have a toddler? If not, is there a clear reason why not? Listen to noises in the background on the phone. (The guy who sold me the cabinet had banging noises behind him. And, he sounded stoned. And, he worked construction - freelance. Check and check. He turned out to be exactly what he said he was.) Pay attention to the email address - is it from a work server? (That's usually a good sign.) Does it include a "name" that doesn't identify the person at all (email@example.com), and if so, does that make sense for the demographic and personality the seller is manifesting? Do the person's accent and literacy level make sense with the other information you're getting (where he lives, the hours he's available, what he's selling and why)? And, finally, is the person just giving you a really bad vibe? I don't mean whether the person is overtly rude or unpleasant - that doesn't really matter. I mean the nameless something that you feel embarrassed to mention because you can't explain why it bothers you. Do not be blinded by the treasure you've found. Listen to that voice. If the person is really putting you off, DO NOT GO. It will always turn out that, when you sit and think about it, the inexplicable feeling is actually the sum of a lot of tiny things you noticed that don't represent normal human behavior, and therefore put you on edge. That doesn't mean the person is a psycho - lots of people are weird, or sometimes behave in weird ways. But people who are lying or have ulterior motives are particularly likely to behave in ways that don't add up. If your gut says you can't trust someone, listen. There will be another art deco mop sink later. (I hope.)