How can I show my stock on currency.poe.trade?

Solution 1:

The "api" icon means that the data has been retrieved using the public API rather than by scraping the forums or other sources.

Yes, you need premium stash tabs to make use of them.

Once one of your tabs is a premium one, put in there the currency you'd like to sell, set the tab to public, right click on the currency (only on 1 stack per type is enough) and select the price of the single orb. Now all the currency that you have in that stash tab will be seen through the api as "stock".

Let's say I want to trade my chaos orbs for alchemy, vaal and exalteds. Do I have to create 3 stacks? Also can I keep those in a separate (normal) premium tab?

Using a normal stash tab (i.e. NOT the currency special stash tab), yes, it is possible. Make 3 stacks, price each accordingly (alch/vaal/ex) and all three listings will appear on currency.poe.trade

A word of warning: the moment you remove the last chaos from the stack associated with, let's say, the vaal price, that price will be removed from the api. The best thing you can do is to have 3 chaoses sitting in 3 separate stacks that you never touch. Unpriced stacks will count towards the "stock" number.