Make sure you are purchasing merchandise from a reputable source.
Do your homework on the individual or company to ensure that they are legitimate.
Try to obtain a physical address rather than merely a post office box and a phone number, call the seller to see if the number is correct and working.
Send them e-mail to see if they have an active e-mail address and be wary of sellers who use free e-mail services where a credit card wasn’t required to open the account.
Consider not purchasing from sellers who won't provide you with this type of information.
Check with the Better Business Bureau from the seller’s area.
Check out other web sites regarding this person/company.
Don’t judge a person/company by their web site.
Be cautious when responding to special offers (especially through unsolicited e-mail).
Be cautious when dealing with individuals/companies from outside your own country.
Inquire about returns and warranties.
The safest way to purchase items via the Internet is by credit card because you can often dispute the charges if something is wrong.
Make sure the transaction is secure when you electronically send your credit card numbers.
Consider utilizing an escrow or alternate payment service.
Source: www.fbi.gov