I posted this opinion after doing some testing with the "Cards in another transaction" message, and I believe it means that either you are offering the same cards to different people, OR someone else is offering those same cards to the person you're offering them to.
I also have used the Trade Transaction tool instead of the Sales tool for the reason you stated. You're not missing anything.
Follow my blog - I Identify as a Card Collector.
“Aslan didn't tell Pole what would happen. He only told her what to do. That fellow will be the death of us once he's up, I shouldn't wonder. But that doesn't let us off following the signs.” - Puddleglum in The Silver Chair by C. S. Lewis