I wouldn't trust the sellers reason for selling. Service history is key on any German car. German cars get cheaper to buy the older they get but maintenance costs always remain expensive.
Could blue smoke be injector seals?
Either way I think you'll be looking at a lot of repair bills in the not too distant future which is why I think the seller is selling.
Has the gearbox been serviced?
As the saying goes, if it sounds too good to be true it most often is!