I am not 100% sure but I believe the NW Parcel Office was a parcel-handling office of the Royal Mail in Northwest London (the only city I am aware of that had different postmarks for different postal offices).
There are two stamps with that design, Queen Victoria Jubilee 4d 1887-92(Scott 116), and King Edward VII (Jubilee style) 4d (Scott 133). My guess would be QV Scott 116 as the KEVII stamp had a larger white margin around the green central design.
Edit: If QV, I wouldn't expect one to sell for more than $4, if KEVII would expect it to go for more than $9.
Is the N.W. Parcel Office an overprint or part of the cancellation or applied by the N.W. london Post Office? I see Gov't Parcels stamps listed in the Officials but not this one. Would this be a different stamp from the ones overprinted Gov't Parcels?