Do I have to give credit to the author

I am a seller as well as a buyer on envatomarkets, especially on videohive.

There is the following question from a buyer’s view:
If I use a bought footage stock (video) to upload it on Youtube, do I have to declare in the public descriptions of that video that it is bought from videohive. Something like ‘© / Name of Author’ or ‘© Envatomarkets / Itemnumber’.

If I don’t declare this, how might Envato see that this isn’t an illegal copy of the video?

Do I have to credit the author of the item in my end product?

No, it’s not mandatory to give the author credit. But we do encourage that if your end product has credits as part of its design (such as a film or TV show), please credit the author and Envato Market. Also, as the author retains ownership of the item, you shouldn’t claim copyright in the item.

Example: If you used footage in your movie that already has credits, we ask that you include a credit. If you use a button graphic in a website, you don’t have to credit the author of the button graphic.