No#1. Try to not use your personal phone numbers on ads. Use those numbers which are not so important to you.
No#2. Set your price little high that you can negotiate your price with customers.
No#3. Buyer or seller what ever you're, don't set your meeting place in a remote area.
No#4. If you're a seller then don't accept your payment in check. Your check may be bounced.
No#5. If you're a buyer then always ask for a valid bill of product from a seller and make a sell letter.
No#6. Avoid cash transactions
No#7. Beware of unrealistic offers