Essentially, Rebellion (developers of SE3) have blocked slightly over 7000 keys sold via 3rd party retailers. Rebellion claim the keys were stolen and sold illegally to online retailers - 60+ retailers, including Green Man Gaming, have been affected. Some of the retailers - like CJS linked above (who are NOT affected by this issue), are claiming that Rebellion are pulling a fast one because they don’t like the fact that the keys are being sold at lower prices. 
This should be an interesting 
storm to watch.