Well off the top of my head I suggest the following.
XP: Based on gold value traded (In this case I feel people may try and artificially boost their xp like trade score so I say put a reasonable max on it per transaction)
Number of orders
Number of items shipped
Or some combination of.
As for what they can do. You would lvl these up the same as commanders currently do.
1. Lower taxes
2. Increased van speed for vans under the Trader (Command)
3. Increased load capacity for vans
4. Better chance of running through blockades, and receiving through blockades.
5. If diplo's eventually can raid/steal from hubs trader would boost defense against that.
6. Increased carrying capacity for gatherer's (not sure how exactly this would work with command)
7. Extra vans recruited under Trader
8. Teleport goods :D
Great idea though.