Vendor resources are being sold as an extension outside of the normal Renderosity license (which usually states that you can't redistribute any part of the product in any way.)

Therefore, vendors are free to put certain usage restrictions on their resources. Common restrictions are:
Listing the item as a credited resource or leaving copyright marks intact
Must change X% of the resource
Must not use more than X parts of the resource
Must only be used in commercial packs, not freebies
etc.

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire