You can only do this with cartel market items.(need to be on a player WITHOUT the item already unlocked, hence, the one that did not equip it first) Steps are:
1.- Open inventory.
2.- Open Collections at the bottom.
3.- Scroll down until you see the item category (armor, crystals, mount, pet, etc).
4.- Flip through the pages to find the item.
5.- When you've found it, click it and press the icon at the bottom left corner.
6.- Purchase.