Thank you @photo-trail! The financial loss was not my main problem, it was rather the fact that there are people on this blockchain who're only waiting for other people's mistakes in order to ripp them off.
Sure, I have no problem in telling the whole story :-)
On this page https://smartsteem.com/buy I accidentally copy-pasted my key into the first field where it says 'Post link':
Then I sent my key to the smartmarket wallet, and the transaction - including the key in the memo - was obviously also displayed in my own wallet.
It's as I said, I was doing too many things at the same time and not focussed enough. That's not gonna happen again :-)