Do I need a license if I resell crypto without holding any money and only charge a commission?

What you are trying to do is referred to as an OTC (Over-the-Counter) Trading Desk. Whether you purchase the coins from an existing purse or from an exchange in real-time when the client purchases, the deal is the same (in our opinion), and in such a case you would be running/mimicking an OTC trade. 

Any OTC trade like business requires that you:

1. Register with FinCEN (Financial Crimes Enforcement Network)
2. Under certain tokens/coins, you might need to have the regulatory licenses as offered by the SEC/FINRA (Please note, this is not our area of expertise and hence we cannot advise on this)
3. Have the necessary Money Transmitter Licenses and/or Cryptocurrency Licenses (where applicable) in the US States from which you are signing up customers and doing business with them.
4. Adhere to various state and federal guidelines (including reporting) for anti-money laundering, counter-terrorism financing, PATRIOT Act, Bank Secrecy Act, etc. 
5. Depending on your flow of funds and transaction specifics, you may require a license in either one state or multiple states.

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