What patch version are you using? I recommend that you install the latest v1.45 patch, that is if you haven't already. I know that earlier versions of the patch had a bug where the merchants inventories didn't update;thus, causing the problem that you described. Another thing to try is to leave the battlefields, and then return a few minutes later. Maybe the merchants inventories will be updated when you return. I hope this works for you.
Good Luck!
