I think that it is a very interesting topic. Maybe you should move it (into bitcoin discussion or mining speculation?) as many people/miners might be interested.
First off, I have no tax advice at all, and you need to consult with a professional tax adviser.
However, from what i understand, W9 form is for payments to YOU (that's what they refer to as payment transactions).
Since you were mining to your Coinbase account, you have many payments directed to you.
Ebay is the same ($20K, 200), so there is nothing unusual about this.
Your own account sending money to coinbase does not constitute payment transaction (as far as I understand), NOR does the spending of your money from your coinbase account.
You get 1099 for payments to you and then report expenses on general tax forms.
It would not be any different if you are a contractor, consultant or a part-timer (although in those cases they typically ask you for W9 beforehand).
Agreed, form 1099 is for payments + the following:
Box 1a. Gross Payment Card/Third Party Network Transactions
Enter in box 1a the gross amount of the total reportable payment card/third party network transactions for the calendar year. Gross amount means the total dollar amount of total reportable payment transactions for each participating payee
without regard to any adjustments for credits, cash equivalents, discount amounts, fees, refunded amounts, or any other amounts. The dollar amount of each transaction is determined on the date of the transaction.