Welcome to the Forum Warartist.
Below is a pic of my 6.35 mm Ortgies. While I guess you could execute a prisoner with this mouse gun, I would think it would be a lot easier with something a bit bigger. These were manufactured immediately post WWI as primarily a civilian CC pistol. Remember that 1920's Germany was a pretty chaotic place and people wanted to carry a small, easily concealable pistol for protection.
If you have not seen it, here is a good write-up on Ortgies pistols:
http://ortgies.net/ortgies0015.htmAs for the price... that is for you to decide. Personally, unless this was Her Hitler's personal pistol (and it had better come with absolute proof), $800 is crazy. For something that was not standard military issue, capture papers add very little to the value IMHO. Have you checked the prices on some of the auction sites like GunBroker? I paid ~$175 for mine at Cabelas.