Off topic:
First, there are things like manuals, guides and tutorials. Do yourself a favor and spend some quality time with them.
Second, I would think it is better to collect all your questions in a single thread instead of opening a new thread for each one. That makes it easier to keep track of your setup details: You told in the other thread that you're playing single player vanilla CIV IV, without expansions or mods.
So now your
answer:
Most important question: You LOSE the bonuses of a resource if trading it away, unless you have more than one copy. Basically, if you have one wheat and trade it against one ebony, you lose the +1 Health bonus from the Wheat, but gain the +1 happiness bonus from the ebony and gain the ability to build war elephants. So that deal is probably worth it - IF you need more happiness or want to wage early war. Even better if you have a second wheat, because there is no benefit from a second copy, unless you trade it away.
How to see the resources?
Again, I don't know about the standard user interface in vanilla, but there should be several screens where you can see all your resources. One of them is in the diplo screen, the one where you actually trade the resources. Others are in the various statistic screens that you can access via the toolbar in the upper right corner or via the Function keys (in the Better BAT mod that includes BUG, I know that you can see the known available resources in a relations manager on the F4 key).
Since you are the one who built the improvements and connected them to your network, you should actually know that yourself, too. Unless your empire is already too vast to keep track of.
Finally, you should be able to see your resource icons in each connected city screen on the right side. That is critical info because cities can only access resources that you connected to them via roads (or harbors, if trading overseas).
