I also tried booking on Thalys, but its the same problem. A cheap fare comes back in the search result but when I try to buy, the system will say its not there. The next cheapest fare is at least double the price.
"The fare you selected on the previous step seems to have been sold out in the mean time."
On the
www.nsinternational.nl website, I found a reasonable fare. When I try to make payment, it says my credit card is not acceptable. Seems like a challenge to book train tickets in advance...