What you described is really no different than buying and selling on a traditional exchange, like Coinbase. It just sounds like you are doing it in-person versus remotely via an exchange. You are buying an asset from one individual and selling it to another individual. No different than what you'd do on an exchange. You will recognize capital gain or loss with each transaction, and this will be reported on Form 8949.
If you want to form an actual cryptocurrency exchange, there are a TON of regulatory hoops you must jump through. Most of them are very expensive, and you will likely have to register with FinCEN. If you form the exchange in the US, you will be subject to a TON of oversight.
Based on what you have described, you are a crypto investor.
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