In that particular case, I'd suggest the seller was within their rights, honestly.

If the code wasn't obtained through any official means provided by the seller, then the seller has no responsibility to honour it, even it happens to 'work' in their checkout.

But the seller was obviously stupidly petty, and that feels pretty pathetic on their part.

They should have just sent your stuff, and taken the experience as notice to replace the voucher code that got leaked, so it doesn't happen again.