Mercenary Shop
The different Mercenary Shops are available by talking to Elusive Shuan in Steppe Vista or Lloyd the Redoubtable in Barrow of Sadness / Phinko the Sage in Lost Wastelands or Bloodthirsty Kyew in Ridge of Darkness. It does not matter for which faction you have the reputation, you can buy items for which you have enough reputation in both shops. The only thing that can only be bought in the respective store are the Mage's Amulet of Reflection.
For a guide on the reputation there is the Mercenary Reputation Guide.
Reputation
Use
500
Torch: Light it to have a Burning Torch, that lasts for 3 hours and is required to have access to the Catacombs.
Scroll of Oblivion: Resets your combos and you will have to find them again.
1000
Toxin: Can be used on Berona Tiger / Crion Zorb to turn them poisonous, making them stronger over all and giving them the ability to use Venomous Bite.
Scroll of Total Exorcism: Removes all hexes. A scroll is only sufficient to remove hexes once, then you have to buy a new one.
2000
Concentrated Toxin: Can be used on Luan Tiger / Battle Zorb to turn them poisonous, making them stronger over all and giving them the ability to use Venomous Bite.
Shuffle Elixir: Changes your combos, but you will already know them and won't have to find them again.
3000
Crystal Toxin: Can be used on Elite Berona Tiger / Assault Zorb to turn them poisonous, making them stronger over all and giving them the ability to use Venomous Bite.
Mercenary Rucksack: Increases backpack space by 12, but only one can be used at a time. Can be made unbreakable using Brass Resin.
Worship
Mage's Amulet of Reflection: Allows you to summon a Warrior to help you in the fight for 5 Mercury Mirrors each summon. The amulets are permanent.
For more info on mounts: Mount Guide.