While some of these suggestions might be a bit obvious, they're worth repeating.
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, 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, ask to talk on the phone. If you're interested in the item they're selling, ask a few questions about it. 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. Remember, you have no idea who you are talking with so you want to find out as much as possible to make a good decision. Use the seller or buyers online post to gauge them. If they are new to the forum with no history the risk goes way up.
Do not 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. 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.