For a Westley Richards double in a Nitro cartridge it does sound like a very cheap deal. (edited - I am assuming it is a "Westley Richards" made rifle by this comment)

Shoot it if you can and also see if you can get it checked over by someone who knows what they doing.

If it is kosher, I wouldn't worry too much about difficulties in getting ammo, I would go for it and worry about ammo later.

Woodleigh makes projectiles. Bertram makes brass.