To check a vendor’s legitimacy begin by:

Calling the vendor’s phone number listed – Do they answer as a business?
Check online with 3 different telephone sources to see if the phone number is listed for the vendor with the  corresponding address provided.
Check online to see if a website for the vendor exists – Does it look professional?
Check the Better Business Bureau to see if the vendor exists on their site – Don’t forget to check the BBB rating.

Click Here to search valid business entities by state.
Click Here to validate the TIN with the IRS.

While one can never be totally sure a vendor is not fraudulent take these steps to try and mitigate most issues.