I feel like a noob asking but planet trades are new to me... This is mainly referring to planetary items such as The Gaia Seed. I have seen people selling items like these but they can't be sent. How is the trade done?

I alert a terra planet.

You colonize, add artifacts and/or use terraformer.

You leave legion, I take planet back.

For things like Gaia Seeds, Hydro-Shifters, Iradiators, Mass Surface Melters, Gamma Crystalizer, Planetary Sentioxes... etc. The path is one of these two.

A:
0. Payment is exchanged either partial or full.
1. Planet is alerted and abandoned by the owner.
2. Colonized by the seller.
3. Artifact(s) is/are used by seller.
4 Seller leaves legion.
5. Buyer invades planet.
6. Done deal
6a. rest of payment is made if partial route.

or B( less common for planet change artifacts This is how reality transfusers and Quasi-Spacial Expanders can be sold.)
1. Alerts an unoccupied version of the planet type in question.
2. Seller colonizes and uses artifact then leaves legion.
3. Buyer invades planet and gets to building.

For planet structures that aren't storage towers:

1. Buyer alerts unoccupied planet.
2. Seller colonizes and places the artis to be sold.
3. Seller leaves legion and buyer invades.
4. Buyer extracts the artifacts using a Mylarai Extractor.
5. If more artifacts are to be sold Seller invades planet, places new structures, and the buyer invades again and extracts them. Repeat until finished.

Hope that helped.