It would be useful to highlight a few ways to avoid scams or what to look out for. I recommend checking these bullets, it could save your wallet!
Always go with your gut feeling and what it makes you feel inside. If immediately the email sounds weird, too good to be true, and/or raises a red flag, take precaution!
Use a search engine like Google, and search the actual potential scam email. You will be surprised that sometimes people or classified ad users will warn others about the scam.
Just don't respond, its not worth it. There are a lot of legitimate buyers out there, so just work with them.
Keep an eye out for listings that have many typos, missing contact information, no pictures, and unusually low or high listing prices. Postings that use all or many of these should raise red flags.
Avoid contacting listings that are repeated many times on classified ad sites. These might not be scam but might be spam. I noticed this on a lot of larger classified ad listing sites. Many contain tons of spam and have no way of really controlling it. Frustrating for people searching for quality listings/products that actually meet their search criteria! In addition, this can be frustrated for people listing products, but then not getting visibility because of copious amounts of spam.
Try them on the phone. If you are worried about a scammer, see if you can talk to an actual real person before even proceeding with anything else.
Deal with people locally! Check out the product/service you see listed and in person is a way better idea than doing everything over email. By doing this in person you can check the quality of the product, and allows for a more a more trustworthy transaction.
Beware of listings that require online transactions, wiring, or fake cashier checks.
Beware of listings or emails that require financial information, social security, credit card information, bank accounts, personal information, and or credit card checks.
Hope these tips help! If you ever want to report scams to the authorities you can do so by reporting them to IC3 or the FTC online.