The problem is not so much imitation or OEM BMW parts. If you're getting anything BMW, from outside BMW, would know anyway that you're getting imitation or OEM. The problem is when you're getting aftermarket stuff and think you're getting real AC, Breyton, etc, at a steal, but you may in fact be unwittingly getting counterfeits. The market is such that counterfeits are now mixed in with originals in almost everything from spark plugs to engine oils.
Even a reputable dealership in Bangsar, which I shall not name, is doing it. Luckily I've been using them for 20+years and the owner will tell me: this is ori, this is imi, etc. Would he tell a walk-in customer? He even has imi rally-class brake lines! Counterfeiting is big at the moment.
Just be careful and use good sense when you buy aftermarket. You would spend a quarter mil on a new E90 and yet balk at spending 15-22K on a good set of the 4 most important things on it? Tyres, rims, brakes, don't compromise. Good stuff there doesn't come cheap.