I think we need to define what is a poor seller. Like njee says, I can only think of the Farish 350 and the Dapol 56 as what would appear extremely poor sellers; they seem to have been hanging around like bad smells for quite some time, the former especially.
But other than that, what?
If something sells out on pre-order or as soon as released, clearly that makes it a good seller; but having stock on the shelves, is a good thing for us, surely, and doesn't necessarily make it a "poor" seller. It means you don't have to rush out and buy a model as soon as it's released, lest it be gone, if not forever, then until the next time it's manufactured, often 4 or 5 years down the line. D8000 is a case in point, from a personal point of view.
The N market, whilst perhaps not as buoyant as we'd perhaps like, I think looks reasonably healthy. By all accounts, outstanding (ultimately) demand for Revolution Class B tanker (a production run, by all accounts, that the "Big 2" would be happy with), their TEA, Cargowaggon, etc. . Farish Class 40 selling like hotcakes, public uproar at least partly responsible for Dapol taking a second look at their 50.... I'm sure there are others.
I'm not sure things are as bad as they are maybe portrayed.