1. So you don't feel bad about breaching a contract. You've made that clear.
2. Not relevant to whether you breached the contract.
3. That's not relevant at all as to whether you breached a contract. Breaking promises for your convenience demonstrates a clear lack of integrity.
4. If it's shady, why are you selling them? Doesn't that make you shady as well?
5. Again, it doesn't matter that you refunded him. I agree that it is far better for you to have done so, as it keeps you from being an outright thief, but you're still in breach of contract.
6. You keep talking about risk. What risk was there? You were holding the ticket until Muxx paid you your money. At what point where you risking anything? If he had defaulted on the payments, you could have kept his money and the ticket, so if anyone was risking something, it was Muxx. Turns out, you lack the integrity necessary to honor your contractual obligations. What should be a no-brainer is that when you agree to sell someone something on installment, and that person follows through, you honor your contract.

Breaching a contract essentially is breaking a promise you made to follow through, but with much more on the line than just your word. Sorry, chief, but your integrity is shot, and your comments insisting that you did nothing wrong just weakens your already shaky position.