Your intuition is your best weapon against getting conned in a sale. While it's unlikely it'll happen in the first place, you'll know when you're talking to someone who makes you uncomfortable. While a little discomfort will often rule out some safe buyers/sellers that you just find off-putting, unless you're selling some rare item you should be able to be picky and still find someone you're comfortable selling to or buying from. Ultimately, you just need to be patient.
If you have a prospective buyer/seller in mind, just get to know him or her a little bit. Ask to talk on the phone. If you're interested in the item they're selling, ask a few questions about it. Questions about usage often lead to answers with personal information. You can use this opportunity to ask a couple of questions about the seller to get to know them a little bit. If you're the seller, encourage the buyer to ask questions about the item. Make them feel comfortable, let them know a little bit about you, and try to find out a little bit about them. This can seem a little awkward but you can often find ways to ask a few questions. You're going to be setting a place to meet, so it's not unreasonable to ask (approximately) where they live and/or work. You can use this information to mention something you like in that area or even let them know you've never been there before. You can ask what the area is like, how far it is from your location, and so on. This shouldn't be a long discussion, but devoting about a minute of your conversation to ask a few questions will help you gain some comfort in dealing with the buyer or seller.
Avoid Online Scams
Forgetting about the numerous Nigerian Princes in need, most scams you'll find when selling online all have one thing in common: they're too good to be true. If you're selling, this almost always comes in the form of a buyer in another state who's willing to offer you your asking price if you jump through a couple of hoops to ship it to them or their relative in yet another location. Generally you want to avoid replying altogether. If you're not sure the prospective buyer is a scammer, just keep asking for information until you know one way or the other. The more B.S. the scammer has to make up the easier it will be to tell they're a scammer.
When it comes to buying, however, it can be hard to tell the difference between a good deal and a scam deal. I've found some very, very cheap stuff on Craigslist before from legitimate sellers who were just in a bind and needed to offload their stuff quickly. Scams tend to show up in the exact same form, so you need to be cautious when approaching a great deal. For the most part, the best thing you can do is get to know the seller (as described above) to get a feeling for their level of trustworthiness. Keep an eye out for red flags, which often come in the form of unusual meeting places and payment requests. If you still don't feel comfortable, however, just make sure you meet in a public place, bring a friend with you, and thoroughly test what you're buying before committing to the sale. Alternatively, move on. You're making a purchase, not a life-altering decision. If you miss out on one great deal, be patient and you'll find another. Rarely will prices go up if you're willing to wait awhile. There's very little you need right now. Being patient gives you a great advantage over the majority of people in the world, in many situations.
Avoid Counterfeit Money When Selling in Person
I'm always paranoid about receiving counterfeit money, especially on large transactions, even though it's never actually happened. Still, it's worth protecting yourself because you only need to be screwed once for it to be devastating. There are a few things you can do to make sure you don't run into this problem.
First things first, ask to be paid in bills no larger than $20. Counterfeit money is generally produced in the larger bills sizes (meaning $50 and $100 bills) because it's more efficient. If you're paid in twenties (or smaller), you greatly reduce your chances of getting counterfeit money.
If you need greater assurance, simply request to meet up at their bank. This way you can meet in a public place, which many people tend to prefer, and then if the buyer does, indeed, want to buy from you, you can go to the bank or ATM with them and see that the money is legit.
Your eyes also work pretty well when it comes to detecting counterfeit bills. The US Secret Service provides some tips on detecting counterfeit bills, so read up if you want to use yourself as a countefeit-detecting machine.
Finally, you can always pick up one of those counterfeit detecting pens. While this will work, it creates a really awkward situation. If you want to be sure, this is the way to go. In most cases, however, the previously-mentioned techniques will serve you just fine.
Utilize Common Sense
If time has taught me one thing, it's that common sense is not very common. While some of these suggestions might be a bit obvious, they're worth repeating.
Do not, under any circumstances, let your emotions dictate the sale. This is not to say that you shouldn't pick a buyer you like and feel comfortable with, but that you shouldn't lower a price because of somebody's financial situation or change the terms of the sale out of pity. Do not rush into selling because you're afraid of losing the opportunity. Be patient. If anything is going to get you into trouble, it's going to be impatience.
Generally, though, the other party is going to be equally as paranoid as you are. It's a little bit scary meeting a stranger off the internet for the purpose of exchanging some valuable. In all likelihood you'll both have concerns, so just be a nice person while taking the necessary precautions and you should be just fine.