The essential character of money is that you transfer it (or control over it) to another person in exchange for some object or service.
There is no way to use money to purchase something and still retain the money.
None of the alternatives are acceptable to you for various reasons
Invest money in a business and defer purchase until investment ...
You can use one of the Bitcoin exchanges and using their API, create something with the following logic:
current_balance = get_balance("BTC")
if current_balance > 0:
print("Sold %s" % current_balance)
Tax questions should largely be off-topic here, as they are so jurisdiction-specific. But nonetheless, a general answer is sort of possible here:
In many jurisdictions, you will be liable to pay capital gains taxes when you make profit buying and then later selling some asset. This tax is calculated when you report your taxes to your local tax authority, so ...