That thread only refers to commercially sold items from Renderosity. Each and every store on the net has it's own EULA and many stores do not allow commercial products to be made based on their items. I know for a fact that many font makers don't allow their fonts to be reproduced and sold. Always read your EULA's when you buy commercial products that you intend to use for resale or giveaways.

Ignorance of the copyright laws won't protect you in a court of law. 

It is a miracle that curiosity survives formal education. - Albert Einstein