Welcome to the Board, Andy.
The ad notes the seller is in Natal, South Africa. The Durban area has gunshops that he can take the gun to. He can tell them you will call and that they may give you, the caliber (bore), condition (any dents in the barrels, etc.,) and even whether the gun is still in proof. He may not choose to permit all that.
Second, as the gun is in South Africa, he is going to have to jump through the South African hoops to get an export permit, if you do not live there yourself (your location is not listed in your post 'tho you seem to be there or going there to check the arm out). If you are South African you will have the hoops to jump through to get a gun permit.
If you cannot get a sound gunsmith to look at the arm, and I believe Bob Henry (google him) may still be operating south of Durban, then you should:
--look at the proof marks on the barrel flats, swot up so you can figure out what they mean and what period they were used in.
--Check the barrels carefully for pits inside (and out) and for dents.
--Is it a sidelock as shown by the distribution of the pins on the locks?
--Is the gun tight which you check by taking the forend off and seeing if there's any play of the barrels against the standing breech?
--Is there any visible space between the barrels and the standing breech (not a problem if the barrel faces and action faces have some space). Being "off the face" should be a deal breaker for you.
--Check if the barrels "ring" by carefully holding them up by the dollshead or what have you and rapping them. Lack of a ring tone may mean the rib(s) are loose.
Likely more checks, and others can add to, or give a better, clearer lists. Best is to get a serious gunsmith to assess the arm.
For determining value, if the gun is sound, John (above) has got the right slant, and a look at the gun auction sights and some of the better dealers may give a clue as to what people are asking for various grades of Lang.
Regards, Tim
