http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&sid=d5570d7646c5...
[(5) Money transmitter (i) In general. (A) A person that provides money transmission services. The term “money transmission services” means the acceptance of currency, funds, or other value that substitutes for currency from one person and the transmission of currency, funds, or other value that substitutes for currency to another location or person by any means. “Any means” includes, but is not limited to, through a financial agency or institution; a Federal Reserve Bank or other facility of one or more Federal Reserve Banks, the Board of Governors of the Federal Reserve System, or both; an electronic funds transfer network; or an informal value transfer system; or
(B) Any other person engaged in the transfer of funds.]
You unambiguously accept other value which substitutes for currency from one person. You equally unambiguously transfer value to another location or person, under multiple legal theories: a) the Starbucks card is one location, the Bitcoin account is another, b) the person holding the card and the bitcoin account may be different parties, c) your business operates as a transport layer between your customers and the company which you are unloading thousands of dollars of Starbucks cards on, and as those are different persons, that means you are transmitting that value.
You might think "But wait, FinCEN narrowed the regulations a bit for 'sellers of prepaid access' specifically to avoid overburdening them." Great news for Starbucks, bad news for you. You are not a seller of prepaid access, which you can verify by reading pg 45403 on of the Federal Register.
http://www.gpo.gov/fdsys/pkg/FR-2011-07-29/pdf/2011-19116.pd...
You are, instead, using Starbucks cards and Bitcoins as a transport layer for value. The government is astoundingly non-neutral about that choice of business.