The devices, which increase the rate of fire of a semi-automatic gun to mimic that of a fully-automatic firearm, may become the subject of a ban proposed by Trump Tuesday afternoon. On Gun Broker, a firearms auction platform, bump stock prices quickly rose.

One listing, with a $399.99 price tag, added “get them while you can guys.” The retail price for a comparable device is less than $200. Other auction prices ranged from just over retail to as much as a $1,000.

After months of pressure to regulate bump stocks following last year’s Las Vegas attack, the White House revealed its proposal as students were protesting another mass-killing, this time at a high school in Parkland, Fla. The same type of gun, an assault rifle, was used in both massacres, just as it was in the 2012 murder of 20 small children and 6 adults at an elementary school in Connecticut.

In Las Vegas, bump stocks were used by the assailant to kill concertgoers from a 32nd floor hotel room—resulting in the largest mass shooting in modern American history. Trump said he directed Attorney General Jeff Sessions to determine whether the devices are illegal, and if so to set forth a ban. His statement may also cover so-called conversion kits used to make sem-automatic guns fully automatic.

Read: How Australia All But Ended Gun Violence

“I signed a memorandum directing the Attorney General to propose regulations to ban all devices that turn legal weapons into machine guns. I expect that these critical regulations will be finalized…very soon,” Trump said. Bump stock prices also rose after the Las Vegas attack, for fear even a Republican Congress might regulate them out of existence. Now, a Republican, pro-gun president has said he might do it himself.

“That is a gross infringement of Second Amendment rights,” Gun Owners of America executive director Erich Pratt said in a statement. He argued that such a ban could be extended to triggers, magazines, or semi-automatic firearms. “While Trump ran as a pro-gun candidate, this action does not appear to line up with his campaign rhetoric.”