Answers should take into account what Tibia as a game/software provides and therefore I can safely say the following.
There is no 100% safe way to make trades with values higher than 1KKK.
If you're asking it and considering real life exists there are many other ways of trading such things as mentioned on the answers.
But, by definitiion, your question has the answer on itself and should not even be allowed.
Perhaps ask what's the highest value and answer with 1KKK though market would be more appropriate.