NO LONGER SUPPORTING
If you would like a replacement, I would suggest jmp's Wandering Trades. It is a more complete and more robust solution.

UsefulWanderingTrader
Spigot Plugin to reimplement Vanilla Tweaks Wandering Trades datapack as spigot plugin with additional functionality.

Plugin adds six new category of trades to the Wandering Traders

• Player Heads (Players from your server)
• Player Heads (Any custom head)
• Hostile Mob heads
• Passive Mob heads
• Neutral Mob heads
• Miniblocks

By default, the Wandering Trader will add between 1 and 3 player heads for trade (assuming your server has that many players), 1-3 mob heads, and 5-8 miniture blocks. Each trade costs 1 Emerald. The miniblocks also require the full size block as part of the payment. For the trade, the player will receive 1 player head per trade, maxing out at 3, and 8 miniblocks per trade, with only 1 trade per available trade possible.

By default Hostile, Passive and Neutral mob heads are combined into a single category.

The player heads available for trade are taken from 4 places (in the default configuration, only 1 is able to be turned off), in the following order:

• Offline Players (all players who have ever logged into your server
• Whitelist Players (if your server has an enabled whitelist)
• OPs List (if your server has enabled ops)
• playerheads.yml (extraheads, this can be turned off in config.yml, and individual heads can be turned on/off in playerheads.yml)

The first 3 options check for duplicates using UUID, this is not possible with the last option.

Installation
Installation is drag and drop. All configuration files are generated on first run.
Notice:
If upgrading from a version prior to 1.3, please backup config.yml and rename or delete it so that a new version with the new settings can generate.

Settings
Settings are fully configurable for price, currency, amount received per trade, number of available trades, which types are enabled for trade (player heads from server, player heads from extras list, miniblocks), individual miniblocks can be disabled. All information about the configuration can be found on the wiki.