https://www.polygon.com/2018/6/20/17484060/valve-csgo-dota-2-trading-blocked-netherlands

Valve has restricted trading in two of its games, Counter-Strike: Global Offensive and Dota 2, in an effort to comply with the Netherlands Gaming Authority’s rules governing loot boxes. The move disables the option for Dutch players to trade items in those games.

In a statement to customers, Valve said it stopped trading in CS:GO and Dota 2 for Dutch players in response to two letters from the Netherlands Gaming Authority (aka the Dutch Kansspelautoriteit), which said that “loot boxes” in those games violate the Dutch Betting and Gaming Act.