I have encountered the stuck disconnector issue mentioned above. Some people have freed them up with some combination of heat and lubricant. Heat guns can be had cheaply.
They can also be removed and cleaned, but be careful.
http://forum.lugerforum.com/showthread.php?t=36571
The issue can also be caused by lack of/incorrect bevel on the transfer lever where it contacts the pin, typically when an aftermarket side plate or transfer lever is new.
If the seller claimed the pistol was functional, you might contact them about fixing it.