One presumes that there is no edict for the 'big three' to sell these but still find it all hit and miss. In my experience, E* always have single/multiple tickets on sale in the bar but as far as I'm aware, they never seem to announce this - I'm invariably on the first train out of Ebbsfleet so tiredness might come into play. When I returned from Spain last month, the 10.06 Girona- Paris train had none for sale, which was a tad disappointing as I've always felt uncomfortable trying to work out the ticket machines, despite the English option and my French being more broken than the Brexit deal. I went for the 'alternate option' and bought a ticket, as I have in the past, from one of the 'rogue' sellers that lurk in said area. Never had problems before but this got me through the gates at GdL okay but failed miserably at GdN. Thankfully, with two cases and a bag in tow, I have a big smile and a Gallic shrug to a local commuter and she kindly used to her card to let me through. There must be plenty of people like myself who wish to cross Paris and I cannot think of any logistical problems of selling said tickets on all trains arriving at the French capital.