How can you verify BMW genuine parts when buying used?

I am trying to understand how people in the UK make sure they are actually getting BMW genuine parts and not replicas or refurbished items being sold as original. Every seller seems to claim their stock is authentic, but it is hard to know what that really means unless you work in the industry.

When buying online, what are the main signs that something is truly bmw accessories? Is it just about the part number and packaging, or are there other details you check like stamps, serial codes, or manufacturer markings?

Some sellers say the parts are removed from low mileage cars, but is there any reliable way to confirm that? Do you usually ask for the donor vehicle VIN or service history, or is that unrealistic?

I also wonder whether buying BMW genuine parts from independent specialists is actually safer than going through official dealerships. Dealers are expensive, but at least you know what you are getting. On the other hand, specialists sometimes seem more knowledgeable about older models.

For those who have been buying BMW genuine parts for a while, what mistakes did you make early on? Have you ever paid premium prices only to find out later the part was not really genuine? And which categories are most risky, electronics, engine parts, or interior components?