Blimey, Jim!...looks like you got your sale already!
Not being nosey chaps...but because I did think about this myself, I remembered that a chap on the BBS had a bit of a nightmare with customs when he tried to import a pistol from the states, although it did get through eventually...and I can't find the thread now, but
this thread makes mention of the fact that it'll be a good idea for the seller to include (in the parcel) a recent signed and dated chrono report stating that the pistol is within UK legal limit of 6fpe.
Wish you both the best of luck with it
Dave