Rails 4: Retrieving Post Notifications (Listener) - ruby-on-rails

Question

I've setup a Purchase Model & Purchases Controller to allow users buy Items from my Rails application via PayPal (IPN).Everything works fine apart from listening for PayPal's IPN answer.How can you actually listen for this post notification properly / what have I done wrong?
Weird is that if I link a return_url to /hook it works, but the notify_url seemingly does not work.Why does PayPal even pass the parameters to the Return URL? - you don't want the User to have to use the Return URL to unlock his bought items.
Opening PayPal in new Tab with Button => works
Passing necessary Item information to PayPal => works
Buy Item with PayPal => works
List item as :status => "Completed" in Database => does not work!
User Model:
has_many :purchases
Purchase Model:
class Purchase < ActiveRecord::Base
belongs_to :tool
belongs_to :user
def paypal_url(return_path)
values = {
business: "merchant#example.com",
cmd: "_xclick",
upload: 1,
invoice: id,
amount: tool.price,
item_name: tool.title,
notify_url: "http://5947992e.ngrok.io/hook" #Test Server
}
"#{Rails.application.secrets.paypal_host}/cgi-bin/webscr?" + values.to_query
end
end
Purchases Controller:
class PurchasesController < ApplicationController
def new
#purchase = Purchase.new(:tool_id => params[:tool_id], :user_id => current_user.id)
if #purchase.save
redirect_to #purchase.paypal_url(purchase_path(#purchase))
else
render :new
end
end
protect_from_forgery except: [:hook]
def hook
params.permit! # Permit all Paypal input params
status = params[:payment_status]
if status == "Completed"
#purchase = Purchase.find(params[:invoice])
#purchase.update_attributes(status: status, transaction_id: params[:txn_id], purchased_at: Time.now)
#purchase.save!
#user = #tool.user
#user.earned_money += #tool.price
#user.save!
end
render nothing: true
end
end
Routes:
post "/purchases/:id" => "purchases#show"
post "/hook" => "purchases#hook"
Item Show View (haml):
= link_to image_tag('paypal.png'), new_purchase_path(:tool_id => #tool.id), target: "_blank"
Thanks in advance for each answer! Please tell me if you need additional information.