楼上的回答恐怕不准确，禁止的应该是素材的转卖，同样来自 FAQ。
What can users do with my content once they purchase and download it?
All content sold on the Marketplace is licensed to the buyer (who may be either an individual or company) for the lifetime right to use the content in developing an unlimited number of products and in shipping those products. The buyer is also licensed to make the content available to employees and contractors for the sole purpose of contributing to products controlled by the buyer.
Each such product built using content purchased on the Marketplace must be a standalone creative work (such a game, simulation, or video) in which content is merely a component and not the primary focus. For example, it may not be a content pack for distribution to engine users.
官方素材应该也是可以自由使用的，不过好像不能以源代码形式再发布。