The only way I imagine this might have happened is if you wrote 0 for inventory count, and some other number for trade count. Is this correct?
The system does not allow you to have 0 as an inventory count, the inventory count represents the number of cards you own, *including* the ones in your tradelist.
If this is what happened, we have to clarify that probably in the UI somehow, when you set 0 for inventory to get a warning that you are actually about to remove that card from your sets.
Yes, that is correct 
I figured the TradeLIST was a list of its own, only for trading - and kinda made sense, because after clicking APPLY, the card would disappear from my inventory (never to be seen again in the tradelist - but I only saw that 50 cards later...).