Amazon hides the identity of its vendors from its customers so you never know who you are buying the goods from.
Thus it is the perfect platform for thieves & crooks to offload vast amounts of stolen or defective goods and this is exactly what has happened .
So it is a lucky dip some are good but just about all of the very cheap are parts that were defective and bought from the factory as scrap for recycling .
And this is often the original factory so they can legally be called OEM and you will note that most say fits X , Y & Z models but almost none say "guaranteed to work "
The aftermarket parts suppliers , Oregon, Prime Line , Rotary & Stens all do a limited line of spares for Stihls
Farmer Tech in China is the largest maker of after market chain saw parts and they do sell direct and over the years I have filled hundreds of parts from them without a single problem.
However the 2 tines I have bought Farmer Tech parts indirectly from ebay both were defective parts & when I gave the vendor negative feedback they simply banned me from their sites ( same person was selling under at least 10 different identities and listing the same part at 10 different prices