online scam (beware)

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i placed an ad on craigslist received an email from a woman named kelly smith, stating that she would purchase my item (diamond stud earrings) for $600...she stated she would send me an extra hundred dollars if i would send it out immediately. she told me to sign up for alertpay.com because that would be her method of payment. i advised her that she could pay me by paypal and she stated she was not going to use paypal because she was scammed. so i signed up for an alertpay.com account and sent her my email information so that i could receive my money. she then sent me a fraud alertpay message telling me that i would receive my money when i send her and alertpay a tracking #. so i went and sent out the item and the addressee was located in nigeria. i called alertpay.com and they avised me that they never sent me a message and that i was scammed. i was so upset because someone in nigeria is going to wearing some free earrings. i tried to stop the shipment and usps said it was too late and that his isn't the first time this has happened. so beware before you make an online transaction receive the money first an if they are from nigeria it may be a scam.. does anyone know any other type of scams to be aware of?