[Update]: Tripwire Interactive has expressly forbidden the use of paid mods in its EULA for Killing Floor 2.
Your Mods must be distributed for free, period. Neither you, nor any other person or party, may sell them to anyone, commercially exploit them in any way, or charge anyone for receiving or using them without prior written consent from Tripwire Interactive.
[Original Story] About a day has passed since Valve’s announcement of paid mods on Steam, beginning with Elder Scrolls Skyrim, and the backlash is mounting against the new system. A petition on Change.org to remove the mod shop has drawn over forty thousand signatures at the time of this publishing, while a number of people are flooding paid mods with bad reviews.
Fears that Valve’s hands-off approach to curating content would result in stolen content being listed were confirmed when a fishing mod was pulled for using assets from another mod without permission. The creator of Fore’s New Idle Animations, a mod that many other Skyrim mods rely on to function, has expressed his opposition against mods being released for money.
Valve has also seen criticism over its policy of taking a 75% cut of revenue.