You automatically put X% of your resource production up for trade. The % is depenant on your trade law. Limited exports, for example, causes 25% of your production to become unavailable to you and it goes out to the market. People may or may not buy it, but you don't have access to it.

You can't change it on a per-resource basis, it's a flat % of each resource that gets traded away, and you can't control it beyond changing the trade law (if you want complete control you can close your econmy and get access to all resources, but that has it's disadvantages as well).

For more, see this: http://www.hoi4wiki.com/Trade

Can I trade away a specific resource? For example, I have an 18 surplus of tungsten, but whenever I want to trade I only have the option to buy more?

Amazing!