Oh no, we sell them all the time and have no trouble restocking. I sold both relatively recently. I'll have to check if we were able to restock it.
I actually think that Saltmarsh was the only one where we didn't go through our original order (we have done so by now, though). Haven't run out of Radiant Citadel or Spelljammer yet, either, but I still consider them in their early days.
I ran Horde in the beginning of 5e and had no trouble with it, but yeah, I can see where people had problems with it. I only ran part of Rise before we moved onto other adventures. (They come out just slightly too fast to play all of them!)