Selling a copyrighted design (stock media usually is) inside a virtual world/game for any currency that can be converted to real money is equal to sub-licensing (illegal).
The “Rolaty Free License” also clearly outlines this.
Paragraph 21.2 -The Buyer’s license to Product in this paragraph is strictly limited to Incorporated Product. Any use or republication, including sale or distribution of Product that is not “Incorporated” Product is strictly prohibited. For illustration, approved distribution or use of Product as Incorporated Product includes, but is not limited to: - as purchased by a game’s creators as part of a game if the Product is contained inside a proprietary format and displays inside the game during play, -- but not for users to re-package as goods distributed or sold inside a virtual world;--
Also take note of following words in paragraph 21.4 -“Without prejudice”- to sub paragraphs 21.1-21.3,
those few words mean you can interpret the following sentences wrong but it will not change the initial conditions set in paragraphs 21.1-21.3. So if you don't know exactly and just do something, that will not be a valid argument in court (if a dispute would end there).
What follows in paragraph 21.4 is a statement that you the buyer get a “non-exclusive” license to “incorporate” the work (to make it part in some product) and have some rights to use that product commercially.
But note paragraph 21.5 - Absent a written grant of rights greater than that contained in section 21 “Royalty Free License”, all other rights or sub-divisions of rights generally included in copyright are excluded from this license and remain the property of Seller.
This means everything in paragraph 21.4 gets excluded if you do not have written permissions to go beyond the limits described in section21 (basically license gets terminated) and all rights remain reserved to the seller (owner).
Anyways, if this all sounds unclear (which it is) and in doubt your specific activity with the work may go beyond the scope of the license, then just seek clarification from the original owner, that's the safest thing to do.