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When selling, do not put your home address in your ad.
To avoid scams, buy and sell with people you can meet locally, in person.
When meeting with someone you don't know, meet in a public place. If that's not possible, have a buddy with you. Also, carry a cell phone; if you feel unsafe, you can call a trusted friend, and stay on the line.
Never give out financial or private information like account numbers, PayPal login, or social security number.
If an offer sounds too good to be true, it is. Walk away!