since coinlab is registered they wouldn't be considered "a person"
Just because a company that has registered as a Money Service Business (MSB) doesn't make them no longer a "person" - which is defined here:
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http://www.fincen.gov/financial_institutions/msb/definitions/person.htmland therefore I am not a money services business or transmitter or exchanger or anything else if I use mt gox services, right?
If you serve as an agent of a money transmitter, then you don't need to register as an MSB yourself.
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http://www.fincen.gov/financial_institutions/msb/definitions/serves_agent.htmlBut Coinlab's customers aren't its agents.
If you are asking if you can use Coinlab to cash out your own coins without you having to register as a money service business then that is a different question. FinCEN makes it clear that as a merchant you can receive bitcoins for goods and services and that you can spend those bitcoins for goods and services -- all without registering as a money service business.
But if you receive bitcoins and then convert those coins to dollars at an exchange, are you transmitting money? I believe the answer is no if you are covered under the limitation for 31 CFR § 1010.100(ff)(5)(ii)(F):
(F) Accepts and transmits funds only integral to the sale of goods or the provision of services, other than money transmission services, by the person who is accepting and transmitting the funds.
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http://blog.bitpay.com/2013/03/how-fincen-guidelines-affect-bitpay.htmlSo if you received the coins from the sale of goods and services and cash those out at an exchange and withdraw the funds to your own bank account, for instance, it appears then that you are not a money transmitter and don't need to register as an MSB.
Now if you received the coins from someone and converted those to cash and returned cash to that person, then that is a completely different transaction and if you would need to register as an MSB in that instance is a completely different question.