Hey Achilles!
Karma points are just little bonuses you can award fellow members if they do something particularly cool, like help you with info, say something funny, help another community member, etc.
There's a Trading subforum that Members can view (which you may have been promoted to since this post), and you just post Trade, Requests, or Giveaways, as well as responding to offers from other members. It's pretty much up to the participants in a trade how you want to handle shipping costs, but usually the sender just gifts shipping unless they're giving away to lots of people. Generally, it's a good idea to start trading, requesting and sharing with low profile plants, like veggies, ornamentals, etc that are less coveted, and better trades and shares will open up after you've established yourself as a trusted trading partner. Any time you receive something from someone, you give them Trading Feedback (there should be a stickied thread explaining that in the Trading subforum), and a moderator will add TF to the sender. That way, the community can tell if someone is new or if they've sent items as planned with others.
This community has been kicking along for years now, and it's usually more active in Spring for the Northern Hemisphere. That being said, we're a pretty small group, and this forum moves at a slower pace than some others. Plant time. :-)
Welcome to STS! Feel free to shoot a PM if you have any specific questions. Moderators and Senior Members are generally good folks to ask questions to.