What is the principal business code for sales?

[VolvoGirl]

The Business Code is a 6 digit number for your activity. Here are the IRS instructions for Schedule C. Business codes start on page 18.
http://www.irs.gov/pub/irs-pdf/i1040sc.pdf

[rjs]

The code that you should use depends on what you are selling. Select from the list of codes at the end of the IRS instructions for Schedule C, starting on page 19. VolvoGirl gave you a link to the instructions.