There are no real rules here (except the usual "follow the ToS" one), but a few of guidelines.
1. Remember the EVE contract system. Depending on the nature of the deals you make here, you might want to use the contract system to back it up.
2. Both EVE and Gaia have rules in their respective ToS's that you cannot trade in-game assets for real-world money or other games' assets. That means, no trading a Harbinger for a KiKi Kitty in this forum. (Not that we can stop you, but if you're going to do it, for your own safety, don't communicate with the other party on either EVE or Gaia. smile )
3. If you're selling or buying, it's a good idea to list 1) how much of the item you're buying/selling, 2) the cost or price you're willing to pay, and 3) the location of the items and/or how far you're willing to ship it (for example, "pickup on Sobaseki" or "anywhere in Lonetrek"). Here is the recommended layout.
Evul Buddha Productions
Items........................................Cost................Drop Off/Sell Location
1k Veldspar..............................1k isk...............Mora
1k Havoc Heavy Missile............10k isk...............Kor-Azor Prime
1x Heavy Missle Launcher .......10k isk...............Sehmy
4. You can use the marketplace to contract work (e.g. "Looking to buy 100,000 units of massive scordite) or to contract yourself for work (e.g. "I have perfect blueprints for these items and will manufacture for you!")
