How about right clicking on a Testificate, it will tell you what he can trade for what. It should items that are uncraftable, and dont have many ingame uses. For example, a slimeball, or a hook(which could be used for a grappling hook, or fishing rod). I dont think they should trade things that are easily obtainable, like iron.
I know, I just wanted to put it into more detail.
Oh, alright then. Good suggestion though, I like the idea of trading for more rare items, though I would still like to see more common items.
I hope Jeb adds a GUI for trading.
It's balanced to have testificates give you obtainable items.