Best way to trade key code

Not really. One of the traders has to go first, usually the one with the lower reputation.

The one with the lower rep goes first, this have worked for me so far.

All you need is a common sense. If the deal is too good to be true - it's probably scam and you should move along.
Most important - do some research about the person you trade with (reputation, library, VAC status etc.)
Take this advice from an experienced trader.

try doing trades with people who have lots of rep and be the first to give until you build yourself a reputation, the chances of been scam from someone with like +130 rep is pretty low.

In person and in a confined supervised space no one can leave for a couple of months or maybe years until everything checks out.

The only thing I can highly advise you to do is NEVER go first when you have an active thread on steamtrades and someone adds you out of the blue, especially if it's an fair/great offer.
I've been added by at least 50 people over the last few months, all of whom got fooled by an impersonator and all of them traded bundle game keys for either a brand new AAA title, or some other game that the impersonator claims is a giftable copy.

Tremor sometimes give ur invalid keys, so dont trade it because u cant 100% sure that they works