Martin, I purchased a water level a couple of months back in a moment of weakness for about $300 and have never used it on an inspection yet. I don't think it will ever pay for itself.
If a house is moving, there will be signs in the walls, doors, etc. and measuring how much it is out of level is not really that beneficial, since there is no baseline to compare to.
Save your money until you see the need and develop a viable marketing plan.
That said, send me an email if you are interested in a neat little toy
Jim