Killing Floor 2 [official site] is currently making its way through Early Access and by all accounts is a pretty good co-op zombie blaster. Developers Tripwire are now planning to add microtransactions to the game, letting players buy in-game items using real money. The initial items will be cosmetic, bought directly or by paying for keys to unlock randomly dropped item crates, though there are plans to add weapons "with new gameplay" in future. These items will be available to everyone on a server if one person has paid to unlock them.
None of this is a surprise - Killing Floor 1 had paid DLC weapons and skins - except perhaps that the microtransactions are being added here before the rest of the game is finished.
"In the future we may be adding weapons with new gameplay for sale, but this will appear in the 'Shared Content' area on the server," explains the FAQ for "The Trading Floor Zed-conomy". "This means that, if any player on the server has a weapon (like the Chivalry Zweihander now), then every player on the server will be able to use it. No-one gets any 'advantage'". This seems an improvement over KF1, where DLC weapons would only be available to pickup by other players on the same server if their owner dropped them.
The FAQ also outlines that some items will be tradable with other players, some can be gathered in-game through play, and some will be created by the community rather than Tripwire themselves. In the latter instance, the community creators will get a cut of the profits from items sold.
This is similar to how DLC worked for the original Killing Floor, and is broadly the same as games like Team Fortress 2. Unlike Payday 2, in which the recent introduction of microtransactions prompted a lot of complaints, none of KF2's weapon skins will change the stats of the items you apply them to.
However, some people are still unhappy and the game has had a spate of negative Steam reviews in the past 24 hours. These are mostly players cross that the microtransactions are being added to a paid (rather than free-to-play) game when said game isn't yet finished and out of early access.
The FAQ states that the decision to add the microtransactions now was taken because Tripwire "view the Trading Floor as a feature that needs iteration just like the rest of the features in the game. Early Access is the perfect place to iterate on the Trading Floor feature with the community." Developer Alan Wilson also responded to the criticism on the game's forum, similarly stating that though there was "Not much time left to change anything before the update," they were listening to feedback and were introducing the market now so that "when the game comes out of Early Access, it is actually finished, with core systems like this ironed out and working the way people want it to."