How to sell: That's not a design problem, it's a marketing problem. You can keep slick watermarked low resolution sample videos visible in marketplaces, YouTube, Facebook etc...
But can you do it legally depends on the licensing of the material you include. Freepik and all other image distributors I know prohibit redistributing their images as is and as a part of other products. There can be some image download sites which have explicit license: "This image is free, use it and redistribute it as you like". I do not know one.
If you haven't got an explicit license which clearly allows redistribution do not redistribute. One possibility is to use images which are made for you for distributing. I guess you have not time to draw them by yourself. A co-operation (have a written and signed agreement) with an artist can be useful.
Warning: there can be criminally acting downloading sites which give images with no license or with a fake license. Downloading pirated stuff from there does not make you free of consequences if you distribute stuff downloaded from them. Before you use an image do image search to see who else has the same image. The real owner may be far less generous than the "download for free" or "buy for $1,-" -site.