Dave,
To expand a bit on Ed's comment, there is a small stirrup hanging down from the rear toggle link which engages a lever hooked to the recoil spring. If this is not engaged when the gun is put back together after being field-stripped, the result will be as you describe.
Disassembly and reassembly of a Luger is a very simple task, but engaging this stirrup can be a bit of a trick. It may involve holding teh pistol upside-down while sliding the reciever into position. I'd say take the gun apart--you should be doing that as a pre-purchase examination in any case--and see if you can't get it back together correctly.
--Dwight
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