According to Wikipedia, "In computing, phishing is the criminally fraudulent process of attempting to acquire sensitive information such as usernames, passwords and credit card details, by masquerading as a trustworthy entity in an electronic communication."
Here yopu will find a few suggestions how to avoid it.

Things You'll Need:
Computer
Open Mind
Common sense

Step1
Be careful.
When in doubt about potential buyer's identity, avoid doing business with them.

Step2
Never send your product before the payment clears your bank. This should be your rule number one. Surprisingly many people do the opposite. Please remember that it is an absolutely normal practice - first the payment comes, then the shipment leaves. Of course, there could be some exceptions, when you deal with long-time and trusted customers, but still keep your eyes open.

Step3
Never agree to return the overpayment.
There are cases when the buyer sends more funds than needed, and instructs the seller to send overpayment back. Never agree to this.

Step4
Be cautious with emails - some of them may camouflage as a real legitimate business.
Though some emails may look real (contain real links, such as PayPal.com for example), the real link behind it may be completely different and point to the illegal domain.
Step5
Never click on suspicious links. See the reasons above in steps 4.

Step6
Never perform online transactions using unknown service provider. Sign up only for well-known providers, such as PayPal for example.

Step7
Protect your computer from viruses. Install popular antivirus programs, such as trend Micro, for example, which will be updated continuously and will be able to protect you from most viruses. There is a lot viruses out there, which can steal your electronic identity.

Step8
Use spam filters in your email software. Sometimes the legitimate email may be stopped by the filter, so check the spam email folder frequently.