You really just have to use your intuition. Most scams are pretty easy to spot but it's easy to mistake a legitimate buyer for a scammer as well. Absolutely everything I have ever listed on CL has gotten at least one scam attempt. The ballsiest one was on a pontoon I had renovated and was asking 7 grand for. Had a party who wanted it sight unseen for asking price, but they actually called me rather than the standard broken english email so I thought it was legit. Had a handful of correspondences with them and it was made crystal clear that this was a cash only sale. They claimed they were coming from northern Iowa but both of the phone numbers they had called me from were from Saint Louis (I'm in KC). They showed up on a Sunday evening with Indiana plates and a blank money order that was clearly counterfeit. They got very upset when I wouldn't accept it, then insisted I follow them to their bank where they could give me a cashier's check instead. Hmmm, still not cash, and remember it's a Sunday evening and they are supposedly from out of state, yeah, not gonna happen.
Another time I was selling a 15 year old pickup with over 350,000 miles on it. Within 30 minutes of listing a guy emails me and says he wants it for asking price but he's in Chicago on business for a few days and would wire some money to hold it and then would pick it up on his way back to Oklahoma. That one sent up all kinds of red flags, but his 50% pay pal deposit went through and a couple of days later I met him at an Ihop just off the interstate where he handed me the rest in cash and we both drove away happy.
When I was selling my old sailboat I couldn't get any legitimate interest. Every CL hit was either a foreign scammer or a random "will you take X dollars" text message with no response after my reply. I tried for months to sell and even sold it twice on ebay to buyers who never showed up to several scheduled appointments. I finally decided to donate it to Boat Angle, but I ended up doing one last attempt on CL. Within a couple of hours of listing, a local number texts me and says it's exactly what he's looking for and is OK with the price but could only meet me after he got off work and he works evenings. It felt a bit shady but I agreed anyway. I know it wasn't the smartest thing since KC is the 5th most dangerous city in America and all, but I met him at a secluded storage facility outside of town at 10:00 PM. He left his truck running and his door open where I could see the Glock 9mm stuffed in the door panel. He did a brief walkaround, never even looked inside, and said "I'll take it" as he walked back to his open door. That's when I said "I see you show up to a CL sale the same way I do". He says "huh?", and I pointed to his piece. He laughs and says "Yeah, can't be too careful" to which I replied "I completely agree" as I lifted my T-shirt slightly to expose the pistol in my waistband. We chuckled and gave each other a mutual nod of respect, he handed me a wad of cash and drove off with my boat.