BTW, are you looking to sell?
As to your question about legality, GB and any other site have nothing to do with legality in firearms transfers. That's up to the buyer and seller.
What GB and Guns America do add is some vetting of members and you have the feedback to help decide how reputable someone is.
I've sold on GB. The trickiest part is finding a dealer you can trust to handle shipping.
Buying/selling on the forum presents the same challenges.
I personally prefer selling online. I don't like meeting a stranger FTF and have no idea if they can legally own a gun, unless they're comfortable showing me an LTC (and I'm not comfortable showing mine to a stranger.)