Transactions in Coinbase and other e-currencies are handled the same as other investments. Sales are reported as Investment Income, with a sale price (or fair value), date, original cost and date, and the gain or loss.
Expenses relating to the purchase of Coinbase would be included in the basis of the asset. Sales expenses would be reported as part of the sale rather than separate business expenses.
The actual sales are entered in TurboTax under Wages & Income >> Investment Income >> Stocks, Mutual Funds, Bonds Other as "Everything Else." The resulting capital gains/losses are reported on Schedule D.
The easiest way to
find this section of TurboTax is to use the Search box at the top right side of
the TurboTax header. Click on the magnifying glass, type in "schedule d",
hit Enter, and click on "jump to schedule d" to go directly to beginning of
this topic.
Or go to My Account
>> Tools >> Topic Search. Type in "schedule d", then click the
topic in the list to go directly to the start of this section.