I simply feel it's a nice addition to include credits when posting.  If someone sees something in an image you create and would like to use it in one of their projects, they can track it down and use it too. I consider it to be more of a service to the "community", not to promote an individual vendor. I do think that if a free item is used, you should give the creator credit. They have donated their time and talents for others to use, it's a great way to say thank you for their contribution (and often - this is the only "Thank You" they get). 

In general, I see giving credits for items that were created by someone else in your renders as a  service to the Poser/Carrara community.  For Carrara users, it's nice to see what can be done with pre-made poser content in CS.  Some of it works well, and other items don't work so well.  It would be a big plus if CS users who do credit content creators would also give an indication of how well the content imported into CS, and how much work was needed to twaek the textures, etc.

IMHO it adds a touch of class to idicate what pre made content was used for a render.

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