Do I need to credit the author of a Royalty-Free asset I bought when using it as part of video and digital images ?

No, you don't have to pay royalties or credit the author if you're using a model purchased with royalty free license. On the other hand, attribution, or giving a credit to the author is required in most cases, if you're using a free model downloaded from here or somewhere else.

For more detailed information about licensing, read https://www.cgtrader.com/pages/terms-and-conditions specifically #20 and #21 paragraphs.

It depends on what you mean by selling rendered images. If you mean commissioned work for the clients, then yes, you can use purchased models for that, however if you mean rendering purchased models and selling those renders on stock photos marketplaces, then no, such usage is forbiden by the terms and conditions of stock photos marketplaces themselves.