I have just uploaded some of these to oga but theres a few more sets available on the site. All licensed CC-BY.
http://www.ironstarmedia.co.uk/resources/free-game-assets/
The "don't sell them.. to other developers" bit conflicts with the terms of CC-By. If the author means that to be an actual license condition, they should probably consider a different license. Nothing wrong with requesting that as a courtesy, but CC-By makes it technically unenforceable.
My project: Bits & Bots
Would that not be an exclusion, similar to a DRM exclusion?
As I understand it, a DRM exclusion is waiving an existing restriction of CC-By, whereas this would be adding a new restriction, which CC-By does not permit.
My project: Bits & Bots