Any resource in the download section is usualy used for learning and personal projects. If you were to take any of these assets and put them into the commecial sector, but not give credit to the creator... You could be liable to plagerism (passing someones work off as your own, not to mention various international copyright laws...)

Real tricky subject talking about the copyright ownership of others property.

Personal/Learning use = OK
Commercial use = Better kiss the creator A** and hope they say you can use for credit to be given to them.

Really its a legal issue that I can only give a little advice on as I studied Civil and Criminal Law as a UK Military Police Officer, so what I say is not gospel...