console.rb

# bundle exec irb

require 'rest-more'
c = RestCore::YahooBuy.new(api_key:'api_key', secret:'secret')
c.get('getCurrTime') # nil error

signature.rb

require 'rest-core/middleware'
require 'rest-core/util/hmac'

class RestCore::YahooBuy::Signature
  def self.members; [:secret]; end
  include RestCore::Middleware

  def call env, &k
    sig = Hmac.sha1(secret(env), percent_encode(env[REQUEST_QUERY]))

    # Move query parameters into path variable, to stop reordering
    env[REQUEST_PATH] = request_uri(env)
    # Add signature to the end of request
    env[REQUEST_QUERY] = {'Signature'=>sig}

    app.call(env, &k)
  end
end

yahoo_buy.rb

require 'rest-core'

module RestCore
  module YahooBuy
    autoload :Signature, 'rest-core/client/yahoo_buy/signature'
    Client = Builder.client(:api_key, :secret) do
      use Timeout       , 10

      use Signature, nil

      use DefaultSite   , 'http://tw.partner.buy.yahoo.com/api/v1/'
      use DefaultHeaders, {'Accept' => 'application/xml'}

      use CommonLogger  , nil
      use Cache         , nil, 600 do
        use ErrorHandler, lambda{ |env|
          RuntimeError.new(env[RESPONSE_BODY]['message'])}
        use ErrorDetectorHttp
      end
    end

    def self.new *args, &block
      Client.new args, &block
    end
  end
end

Updated: https://gist.github.com/godfat/91e15c625e9b7150b1f3