Sorry if this is posted somewhere, I did read as much as I could trying to find these answers but I couldn't find anything. Feel free to point me to a guide if it exist.
What is the process for trading? I assume I post something I want to sell with either a price set or asking for offers. Let's say I get an offer I want. After that I'm clueless. How do i trade? How do I receive the FG?

This should help you, my friend — all the information is included there.
https://forums.d2jsp.org/topic.php?t=79841247&f=34&v=1

How to Trade on d2jsp and many others
https://forums.d2jsp.org/info.php?p=10

Thank you

Please be patient, your repay mediator will contact you as soon as possible.

This is all Player to Player trading site.

You put up a topic in the proper trading ground for the trade you want to make,
other people eventually spot your topic, (You may bump your topic once every four hours in most forums, 24 hours in some)
if they are interested they will post in the topic,

from there you initiate the trade player to player via private message.
*Never post any gamename or bnet in public. This takes place in private message with the buyer/seller. (to verify both its the correct person trading)

-------------------------------------------------------------------------------------

You can also search for the trade you want to make:
1: Click the Iso section in the trading ground of your game if there is one,
2: Input "the name of the item" you want to sell in the search bar

This will give you the list of topics searching for your item.
Post your price in forum gold, in their topic.

If OP is still buying the item he will eventually message you if he finds your offer fair to his ideal, if both of you cant find a deal, try next buyer.