If you can't go into further detail, then why even bring it up at all? Either it happened and is verifiable, or it didn't and you made it up to try to bolster your point. If it DID happen then we'd all like to know about it since it's pretty important information.
"Some guy on the net said" is pretty poor evidence, and if you are going to take a position then you need to be prepared to back it up in fact if you hope to be believed.
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Glock 19, if your statement is " I know a guy who was arrested ( or escorted out, or whatever) FOR selling an 18 year old a handgun in a private sale" , then are you not saying that it is illegal? And if it happened, tell us the details. Thanks