Long Beard is right, thats deffinately an S-4.
They normally are made of rather soft alumiminum, and do not have a pin depressor in the ASA. You mush use a remote, or an older on/off tank valve. Most functional problems are caused by the MASSIVE hammer, which simply weighs too much for the gun to cycle properly. Try using straight liquid CO2, lightening the hammer, or and HPA tank with a HIGH pressure output (1200+ PSI).
A good teardown and review can be found here:
Have Blue reviews the Qian Wei S-4
I know at least 2 people have actually made these cycle correctly, but it takes a good deal of work. Great wall-hanger or conversation peice though.
Hope this helps. Good luck on the sale!