www.gsmstock.com is not a scam site itself, however its users may or may not be honest. The site itself is genuine, though obviously there is no way that the people who run the site can verify each person's legitimacy 100% (despite all their verification processes).
Just like any trading board, you have to do the investigating yourself. Are there scammers on that board? Of course! However you could say that about any board out there. Alibaba, tradkey, gsmstock, etc. will have both honest and dishonest sellers. You have to investigate each seller and judge for yourself.