I'd question if it had its running in service at all tbh. It certainly wasn't carried out at a BMW dealer, else it would still have been logged on the system. I would ask where the salesman got the information that the running in service was carried out at 12 miles - i sincerely doubt any BMW dealer would have carried it out at that mileage, and it is *far* more likely that the original owner forgot/didnt bother to bring it in at 1200.
What this means in any case, is that the engine is unlikely to be unwarrantied. As such the price should reflect that. If the oil WAS changed, then the chances are that you'll be ok, as these are pretty bulletproof engines. If it wasn't however, then the engine will have run a thicker, non-performance oil for the first 12,000 miles of it's life, and there is also a danger of swarf build-up during the running-in process, that would have been present during that time and not flushed through at 1200 miles.
I have to say my gut feeling is that the oil change was not carried out at all. Certainly BMW UK's records show that, and as such they will not honour any warranty claims on the engine, despite what the salesman claims.