item.json

{
  "name":"Calendar",
  "unit_price":15.00,
  "expense_account_id":"5072d350d57debd2b900001d",
  "income_account_id":"5072d350d57debd2b900000a",
  "description":"This is a test calendar item that will be sold on the online store"
}

item.rb

class Item
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name, type: String
  field :active, type: Boolean, default: true
  field :desc, type: String
  field :unit_price, type: Money

  has_one :expense_account, class_name: "Account"
  has_one :income_account, class_name: "Account"

  validates :expense_account, presence: true
  validates :income_account, presence: true
end

item_controller.rb

class ItemsController < ApplicationController
  skip_before_filter :verify_authenticity_token

  before_filter :authorize

  respond_to :json, :xml

  def create
    @item = @current_user.items.build(params[:item])
    @item.unit_price = Money.parse(params[:item][:unit_price])
    @item.save
  end
end

In order to fix this -- I needed to reverse the relations between items and accounts