If the video clips you're trying to download include a watermark, you're downloading the lower resolution previews, not the actual files.

After completing the checkout process, the products will be added to the "My Downloads" tab of your account where you can click the corresponding format button to begin the download of the unwatermarked clips. (You can see this article on how to download the effects you've purchased)

If you're downloading free products, you'll still need to add them to your cart and complete the checkout process for them to be added to your account.