You may find the specialists in "The King's market" you will need rubies to upgrade/activate them, so this is basically for ruby players.
Some of the specialists are permanent, and some of them are temporary (effect is enabled only for a certain time).
Transport capacity (permanent)
As the name says, you may guess that this will increase your market borrow capacity. Upgrading this item, will allow you to transfer more resources between castles and outposts. Every level you upgrade gives you 7 more storage per market borrows.
Example: If you have 20 market borrows, each with a 200 capacity (total of 4,000) when you upgrade to the next level each market borrow will have 207 storage capacity, giving a total of 4,140. (The maximum upgrade is up to 376 per market borrow).
Although you have to pay for every upgrade in rubies, this item is relatively cheap, and will be permanent ,so it worth doing it from time to time to expand your transfer capabilities.
Marauder will boost your glory and resource looting. It is a temporary feature for 7 days only. IIf you are a non ruby buyer, this thing is a total waste of rubies..
Overseer is used to increase your production rate by 25% for either stone wood or food. It is a useful feature if you find yourself low on resources often and want to speed your production up. If you are a non ruby buyer, this thing is a waste of rubies. Even if you are in a state of war and need resources for repairs, other alliance players can always send you the missing resources.
Bribe the tax collector (temporary)
Bribing the tax collector will increase the amount of coins you receive while collecting taxes. This is a total waste of rubies, to gain more coins simply keep attacks on RBC castles (in all kingdoms) and get coins.