Sell-worthy goodies include ammunition, non-combat pets, guns, and fun, quirky gizmos that are the characteristic trademark of the engineering profession.
As a hunter, I definitely benefited from the fact that my engineering guildmate could often provide ammunition, guns, and scopes. I helped out with mats, of course, and I really liked the hand-made aspect of the weaponry.
Some of these items will require you to max out your engineering, so check out Lisa's guide to pushing your skill up to the last.
If you're thinking about taking up engineering, or are an engineer in need of some cash, The Khorium Toolbox has a very thorough guide that you can't afford to miss.