Several beginners questions

[Enkidoh]

Firstly, you have to hire a retainer before you can sell items on the Market Board (if you've been doing the storyline up to your current level you would have gained this ability - if you're not up to this point and have no idea where you are in the story, check your Journal under 'complete' for Main Scenario and post which quest was the last one you finished, so I have an idea about where you are and to point you in the right direction for the next one.) - there's no way around it otherwise. Retainers are also helpful for storing items and gil as well.

Secondly, there are posting restrictions on the forums based around your character's current level - your in game level dictates how many posts a day you can make and how many new threads you can make (if any). This was an attempt to stop spam posts and threads by new members at the start of service (as during the Beta it became apparent that as more people joined, the more chaotic the forum became, and with all the trolling going on SE thought it was the only way to bring some semblance of order to it). It's possible as time goes on they may lessen these restrictions, but there's no guarantee sadly.

Finally, if you have items you can always sell them to an npc for a small amount of gil (find any npc shop, shown on the map with a bag icon), and when the item window opens up, instead of selecting the shop's merchandise, open your inventory and select the item you want to sell, and click on it. It will then ask you how many you want to sell and after confirming it, it will automatically sell the items for you.

Hopefully this has helped.