Probably two options;
If you shipped it the mail order form would have the address and details of the buyer. Lodge a claim with small claims court although this might not be worth it.
Name the user on here, someone may know them personally.
Just a note, but if you didn't do a s43a mail order form, I wouldn't pursue this matter in any legal capacity. The punishment for contravening Section 43a of the Arms Act is up to 6 months prison and up to $10,000 fine.
Probably best in future you receive payment before shipping unless you personally know the buyer.
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