There is original equipment (OE) from the dealer.
There is original equipment manufacturer (OEM), ie Bosch, from any number of sources.
There are no-name parts that are sold as "will-fit" that don't claim to be OE or OEM. That always has more risk than buying something that is tested and/or approved. Sometimes will-fit parts work, ie there are several transmission fluids that are sold as 'Dexron equivalent' or 'suitable for applications calling for Dexron' that are not certified to meet the Dexron standard (which costs them money), but that have been reported to work OK. You are buying some risk along with your parts in that case, though.

There are counterfeit parts floating around for both OE and OEM. BMW has had problems, as have manufacturers such as Bosch, Pagid, Delco, Autolite, NGK, Champion, and so on.

The lowest risk is to buy from the dealer, but that also has the highest cost.

One way of managing risk is to buy name brand aftermarket parts, from a supplier that you know, and who buys from a distribution network (his supply chain) that you trust. In both cases, it is worth inspecting the parts upon receipt, particularly high risk parts like plugs, electronic components, oil, filters, etc. The counterfeiters aren't creating a market, they are selling into existing markets where there is some volume.

Another thing to think about is what the consequences are. Failed sparkplugs can have high consequences. Using non-spec transmission fluid or any filter can have high consequences. Poorly made wiper blades probably just wear out faster.

Lastly, buying off eBay is the highest risk overall.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue