Properties and identifiers for a merchant item. Very similar
to a FulfillmentItem, but no FulfillmentNetworkSKU nor an
IsActive flag is needed.

The item condition is the concatenation of condition type and
subtype. The exceptions are NewNew becomes NewItem and
RefurbishedRefurbished becomes Refurbished.

Unknown is not a valid condition to use when registering items
for the Amazon Fulfillment Network. It is used to indicate that
the condition wasn't read correctly or was unavailable.