Since the Pre-Order cars cannot be bought or sold via the auction house and the last time some of them (such as the DB11) were on the forzathon shop was 3 years ago, I suggest either putting them as seasonal rewards or putting them in the Car Collector Horizon Life category every 20 ranks or so.
While I do understand the supposed exclusivity, this is not something new, as the Porsche 911 PO is available through the Eliminator Horizon Life category. Besides, most people trying to get every car in the game have at least some of these cars. If you wanted to make them solely exclusive to those who preordered the game, they should never have been available through any other means. Since some of them were, however, it is upsetting to not be able to get every car in the game.