Just to be clear. The marketplace is not a curated store where items are vetted for quality or functionality.
Microsoft do check aircraft but it is for things like:
- the aircraft must be de-miled with all ordinance, guns and gunsights removed (even if the same aircraft is available for MSFS externally on sites like Orbx with weapons and ordinance)
- the marketplace entry for the aircraft must meet the marketing teams specifications for image size, content and type etc
- the aircraft needs to load up in content manager correctly
There is no quality check, it does not even have to be flyable.