I think one of the problems with the used toy train market is that there is so much product to choose from that people get a little numb looking at it all.
That said, when selling your trains people have to understand that unless a buyer is emotionally attached to an item they will just as soon wait until a new item is available before they take out their wallet for anything close to retail.
I have made a profit on some items that I bought, but they are far and few between. On the most part when I buy something it's to have fun with it. So when I'm done playing with it I trade it for something else, or sell it, but I expect to take a loss on it regardless of it's condition.
So in short, until I figure out a way to take them with me they all remain just toys.