I used to buy those kits all the time - still have my Broomhandle Mauser. Got some of the blank-firing MGC gun kits too. These days I'm content to buy the already-built airsofts. Got plently other models to build. (Also, got plently of real guns to play with )
The engineering that they did for the these models to work is impressive. The mauser has sold already to a person in Belgium, that was guick. I removed the BIN from the Derringer maybe it will fetch more.