This is heavily based on Useful Traders, please check out their mod first as I love the ideas and additions but it was too much to get a whole server to agree on.

This is a lite version that only adds the clones of each trader type to the sale inventory for 100 gears, and some extra easily sourceable materials to the buy inventory. This does not remove any existing trades, and should be compatible with every mod that adds more items to traders.

What this does exactly:
Adds a copy of each trader type to their own sale inventory for 100 gears to allow for moving traders into a central location at the cost of both finding the correct type, waiting for rotations, and having 100 gears.
Adds some sourceable materials to a few trader types. Agricultural traders will now buy your excess for cheap. Some other traders will now buy red clay or cattails for cheap.
Adds roughly 10 to 15 gears to their default held gear value, raising the average from 20-35 to 40-55. Luxuries trader holds the most in line with vanilla.
Adds two more sale and buy slots to agricultural (10 to 12) and commodities (8 to 10) traders. 

This is really only meant for a personal server, but if you like it, feel free to use it.
If you choose to use it, please comment issues you encounter with trader inventories, or balancing feedback. If you want more in general, please, again, checck out the mod linked above.