Well, good idea...

But actually, the only thing is needed - "safe" items transfer btw customer and traders.

Could we just rent "safe cell" with numeric-code lock at the bank and put stuff inside.
Now put ingots there and money and mail tinker the code.
Tinker opens cell, does his work and puts stuff back.
Mail code to chanter to allow him to enchant gears.

At the end you opened cell and found it ... empty - someone robed me! :-P. Well, just kidding, normally, you should find your order done inside.

Regards,
-guinness