He told it is really unpredictable how sales develop. Some items printed in one thousand copies still are in stock after twenty years and he has no idea if they ever will sell. Others sold so fast he had them re-pressed immediately. He has to be careful with investing and always looks for top quality original source material.
Anybody looking for items markes as sold out on his website are encouraged to contact him directly. Chances are he still has a handful of copies in stock.