Is cross-region trading/gifting a bannable offense?

Don't really think so. You can trade any game for any game, without getting banned. Paying for getting games gifted from overseas is, I think, a bannable offense, but I've no idea how they would even trace it. Probably if someone gifts too much or trades too much for nothing?

The only problem you may get with buying a cross-region game, is that you may not be able to activate or play it, usually some russian games.

Reddit is completely unreliable. Just a bunch of kids thinking they know everything there is to know about everything. It's definitely not a bannable offense. And that's simply because there are region blocks in place so you can't play it in another country. If you use a VPN to temporarily change your region and trick steam into activating your game, you can get into trouble. But that's because you're in breach of the Steam subscriber agreement, I doubt they would "ban" you seeing as the game was paid for.

Also, I'm aware that some games don't have these region restrictions. Valve would stop people from being able to activate these games if they had a problem with it.

Best price in my region

<eraseline>$49.99</eraseline>

$12.49 USD

my region 12.49€

I get games gifted from the US/Ukraine all the time.

I have several steam friends from ex-USSR who trade cheap games for those that are locked in their region. It's always a lucrative business for both of us. You just have to be sure that the game you are trading with a russian will be available in your region. For example, do not try to trade Max Payne 3 or WItcher 2 as they won't start outside of Russia.