Tickets won't be able to be sold any more than they can be now; tickets are non-transferable, and any such tickets listed on Ebay could, as they would be right now, taken down.

I fail to see why "criminals" are of any relevance here. As long as a person is not barred from flying, or is not supposed to be in jail somewhere, are they not allowed to travel freely around the Schengen Zone (whether by air or surface) just like any other European citizen?