sorry to dig up an old thread, but since this was link #1 on a google search (for something else of course..), thought i'd chime in and keep some poor bastard out of hot water.
You cannot pay tax on transactions involving pot. Not yet anyway.
However, you MUST pay income tax. Income to you from whatever source, donation, reimbursement of expenses, whatever you want to call it .. it's income, and to be legit you need to report and pay tax on it. Remember Al Capone?
To really be legit it can get a little murky here depending on what you are doing. If you are making worthy enough cash then you should really have a CPA who specializes in this field.
What really screws you is paying the income tax on this and not being able to deduct your grow expenses. Being federally illegal, you cannot deduct one single dime of it, not even as Cost of Goods Sold.