Selling cards I don't own

I have woke up to 2 different transaction requests for cards not in my inevntory but behaving like the were.

Most recently I was paid for 2 cards I was selling and one of the cards wasn't one that I had or ever had.  I had to issue a refund.

The other was a trade deal.  On of the cards requested appears to to have been part if my inventory.  It allowed them to select a card I didn't own as trade fodder. 

I'm wondering if anyone else is seeing something similar, or how I can stop this from happening in the future.