In June 2018, Valve had sent OPSkins a cease and decist order for violating the Steam Subscriber Agreement.
Point number two of this cease and decist order demands that OPSkins, "No later than June 21, 2018, cease use of Valve intellectual property (including trademarks, gun models and images from the game CS:GO) on any OPSkins property or media".
The decision to once again include CS:GO items on OPSkins' website goes against this demand.
Following Valve's communication, all OPSkins Steam accounts associated with trade on Steam were disabled on 21st June 2018. But now almost a year later the website seems to have restarted activity in the Counter-Strike space.
The last time the digital store went down, reports suggested that users suffered losses of just under $2M. The online marketplace OPSkins was heavily preferred by users about a year ago for the exchange and sale of all their in-game items. This was due to certain benefits it offered over the Steam Community Market, such as lower commission on item sales, the lack of a price cap on trades, an option to cashout and a full time customer support system that claimed to have an average response time of less than five minutes.
Though Valve took no direct action against them for a long time, it was the introduction of ExpressTrade, a service that enabled users to trade CS:GO items back and forth quickly for free, that prompted them to send the notice in question.
Back in March 2018, Valve had implemented a seven-day trade cooldown to CS:GO trades, in order to combat fraud and misuse of Steam Accounts. As a reaction to this, OPSkins in June 2018 launched ExpressTrade, a service that enabled users to trade CS:GO items back and forth quickly for free, thus enabling its users to bypass the cooldown rules laid down by Valve.
Image Credit: Blog.Counter-Strike
At the time of writing this, there is no official communication from Valve about OPSkins' re-entry into CS:GO trading.