Actually, when you transfer tokens out of your possession to someone else, that transaction is a taxable event. When the tokens are transferred out, a basis cost is calculated and the difference between price at time of transaction minus the basis cost is your transaction's taxable value. Uncle Sam only cares about individual transactions if they exceed a certain dollar value (ongoing legislative, regulatory and legal battles to determine this amount). Typically, the only thing Uncle Sam cares about is the net profit on a particular token you own/have owned over the tax period.
ADDED: BTW, the basis cost is calculated based on your declared accounting method: LIFO, FIFO, or HIFO. But you have to stick with the declared method for all of your crypto and you cannot change accounting methods on a whim.