Three days after you complete Leif's last quest, the Galaxy Island Quests begin. After you rescue Vorlen and the Mayor from Galaxy Island, Tomti's sister, Rem, gets into the visiting merchant rotation. She brings all the items from Galaxy Island and sells them at a Shopping Station. ( I'm assuming you've built Lief's post office and purchased a Shopping platform.)

Purple Nettles are expensive, but well worth the price. They grow all year long, and blossom every few days. I always delay selling them till I have a big stack,( ( over 500), so the price doesn't drop much.)
I think I got about 400,000 petals for my first sell. Rem shows up about one out of five days at your visitor platform. She has the Nettle seeds about half the time,so buy as many as you can afford.

Rem also sells Grape and Artichoke seeds - lots of fun new recipes.
If you're trying to get the "soup" Steam achievement, the Nettle soup might not be necessary - not sure about this.