I get where they are coming from, and I respect the amount of publicity they are garnering towards their cause...however I feel that their focus is misplaced.
The organization of this form of protest is a total mess, for starters. Providing an option for people to sell their violent games confuses their message and lends to the idea that these violent games have some form of value. So do the donations represent people dedicated to the cause, or simply trying to make money...or do they just humor the organization? I don't even want to guess what will happen to the donations (destruction would send a bad message of tolerance).
Videogames being a cause of aggression is a topic that has recently come up due to new found evidence, and I feel that it may have merit, but so do arguments against the evidence. In the end, I still hold firm that bad guidance leads to violence (teaching developing minds to distinguish fact from fantasy, how to communicate with others, the punishment for violence...). However, a violent game could have malevolent effect on a person...especially young people with improper guidance. To that, I remind others of the ESRB rating system and why it's there (and still groan when I see parents buying it for young audiences. Seriously, protesters against violence in games need to address that issue before targeting the games themselves. They are trying to tackle a large issue without taking the small steps).
The smart thing would have been not to mention the Sandy Hook incident at all, and only address it publicly should external audiences bring the incident up first. Exploiting a tragedy to further an agenda, even peripherally, undermines an argument's ethics.
In the end, I enjoy all videogames...provided that they have a point beyond mindless violence. Then again, that is because I see value in games that employ violence to further a particular theme...others may not be able to, and that could be cause for concern.