Noob looking for info

Like the previous person said. Impersonation is rampant. In case that someone adds you always confirm that the person that you're talking to is the same, by clicking on the Steam icon of their steamgifts profile.

signs of a scam

Mostly just don't rush. Common sense can filter out any scams. If you stick to very low levels initially (bundle games, and the cheaper kind), the chances for scamming is particularly low.
But really, if you see someone offering a relatively pricy game for very low amounts, or if someone just befriends you out of the blue and tries to sell you something that sounds good, or just drops the "I'm gonna get paid in x days, can you still send the key" line, you can pretty much tell it is a scam.

Oh, and if you mean by key trade that you want to trade keys for keys, also check barter.vg. There you have to make pre-arranged deals which both party has to accept, so all trades are logged. SG in those cases is used for rep tracking, because this reputation system on this site here is still the best anyone could come up with. (It can be exploited or cheated, but in a limited manner.)