bartering for heals

perhaps its just me that is not understanding this. but help barter did little to assist me in what to write at the healer. I wanted to try heal disease yet none of what I attempted worked. in my posession a gem(worht approx 2500) attempted for instance; barter emerald for heal disease heal disease for emerald(gave a response you do not have that item) Now I attempted many more, but I still did not manage it... As well as the fact that very many merchants do not accept any of the things I offer them for items makes barter quite frustrating. And its not for the lack of trying different things at different merchants.. Just a tad frustrated and a few less hairs on my head....Hope you can help me out with a clear syntax even us mortal souls can understand..