I think the seller should have made it right the day you called. They know there's always 2 mags in a civilian gun purchase, you were their customer and you didn't get what you paid for.

Yes the S&W CS guy was wrong for having an attitude, but I guarantee he gets the 'missing mag' call about 30 times a day. Sometimes you just let some steam off.

What really makes me suspicious is the seller 'finding' a mag after half a year and knowing it was your mag. Of the hundreds of guns they sell, they determined this is your mag. I think word got back to them or they saw this post or something. Is it possible that they found a mag and remembered you called once 6 months ago and said you were missing one? I suppose it is.

No matter what I would be upset the seller didn't take care of his customer. It's a $25 mag and he was selling you $2000 worth of guns in one sale. And you are a regular customer.
You should have been immediately taken care of at the counter and they could work out what happened later.