You may be legally obligated to pay taxes after selling some gold to your buddy, but that doesn't necessarily mean the law is enforceable in that situation.
You can also sell firearms off-the-record to a 'buddy' or a shady character, even if a license or transfer fee is required by the ATF.
You might grow pot at home, sell it to some friends and not tell anyone about it because it's 'harmless'.
You are still taking a risk of imprisonment every time no matter how small you perceive the risk to be.