I haven't found the members on this site to be as big into trading as some others. What I usually do, is go hunting for something that I want (in the plant database). Once I find someone who has it AVAILABLE (many members list their inventory on here but are not into trading, and the site specifically asks that you do not contact members asking for trades if the do not actually have it noted as available for Trade or Sale). If I find someone, I look at their list of WANTS to see if I have anything to offer them that they are seeking, and I Treemail them directly to tell them what I am seeking and what I have to offer that they are seeking. I have found that this works more consistently than just doing a post but it's very time consuming (I like to do this in the winter when I am bored out of my head).
Negotiating is something you learn over time. Generally though, it comes down to communication - If someone asks me for stuff, and I have it available, I try to put "my value" on the items they are seeking - if they're seeking a plant that I have but am not attached to, it has a lower value, so therefore I can be more generous with that than I can with a favorite, something rare, or something in high demand.
To really answer your question would take a long time. So why don't you take a look at my have list for example, and tree mail me to ask me for something in trade and we'll do a little exercise. We can talk more about packaging then too.