Just take it slowly now.
There are lots of options for each item, be it stats, animation options, mercandise, twohanded/polearm/onehanded and so on and so on. Just go read option by option (on the left side of editor) and you will spot option that says "merchandise".
So, scroll down the items list, find desired surcoat (surcoat livonian master, or whatever the name is), and click "yes" for mercandise option. You will see that its turned off by default for some items (these items will never be seen in merchants inventory, so, never can be bought, only acquired through loot).
Play with it a bit, it tends to be very very interesting piece of editor. I made my whole modification for 1257 ad Feudal edition just by playing with Items editor and Troops editor. And some other stuff too but this is already off topic.
1. items list,
2. mail with surcoat master,
3. merchandise -yes-,
4. wait for a couple of ingame days,
5. buy when you see it.