A script to automatize what you already do by hand anyway on wg gesucht (i.e clicking on the ads and sending your saved message)

wg-gesucht.rb

require 'rubygems'
require 'mechanize'

DEBUG = true
SEPARATOR = '========'

BASE_URL = 'https://www.wg-gesucht.de'
LANG = 'en'
CITY = 'berlin'
# this is what you find in the url when tweaking the filtering of the results
FILTER = 'a6349c9d478d85ff9590d585f4dd1aa689187c5117790d1f43'
URL = "#{BASE_URL}/#{LANG}/wg-zimmer-in-#{CITY}.8.0.0.0.html?filter=#{FILTER}"

def log text
  if DEBUG
    puts text
  end
end

def pretty_log text
  log SEPARATOR
  log text
  log SEPARATOR
end

def login page
  page = page.form_with(:id => 'login_form') do |form|
    form.rsLoginEmail = 'YOUR_EMAIL'
    form.rsLoginPasswort = 'YOUR_PASSWORD'
  end.submit
  page
end

def logged_in? page
  !page.link_with(:text => 'Sign Out').nil?
end

def next_page agent, page
  next_page_link = page.at("//ul[@class='pagination pagination-sm']/li/a[strong='»']")
  if next_page_link == nil || next_page_link.parent['class'] == "disabled"
    nil
  else
    agent.click(next_page_link)
  end
end

def get_id link
  link.href[/.*\.(.*).html/, 1]
end

def random_sleep low, high
  sleep(rand(low..high))
end

def clean_links links, contacted_ad_ids
  # reject duplicate, nil links and ads already contacted
  links.uniq! { |link| link.href }
  links.reject! { |link| link.href.nil? || contacted_ad_ids.include?(get_id(link)) }
  links
end

agent = Mechanize.new

agent.get(URL) do |page|
  log "Login..."
  random_sleep 0, 2
  home_page = login page
  if !logged_in? home_page
    log "Error with login, aborting"
    exit
  end
  log "Logged in!"
  log "Retreving already contacted ads..."
  random_sleep 0, 2

  # get urls of already contacted ads, with high number of contacted ads, check if the page is paginated or not
  contacted_ads_page = agent.click(home_page.link_with(:text => 'Contacted Ads'))
  contacted_ad_links = contacted_ads_page.links_with(:class => 'fav_klein', :href => /^((?!javascript).)*$/)
  contacted_ad_ids = contacted_ad_links.map { |link| get_id(link)  }

  # get links for every ads that match the filter (need to loop through pages)
  page_nb = 1
  links = []

  while !home_page.nil? do
    pretty_log "Page #{page_nb}"
    random_sleep 2, 10
    page_links = home_page.links_with(:class => /ansicht/)
    filtered_links = clean_links(page_links, contacted_ad_ids)
    log "#{filtered_links.size} new ads..."
    links += filtered_links
    home_page = next_page agent, home_page
    page_nb += 1
  end

  current_url = agent.page.uri.to_s
  if current_url.include? "cuba"
    "WG-Gesucht busted us! You need to go to #{current_url} and enter a captcha"
    exit
  end

  log "#{links.size} messages to send..."
  random_sleep 0, 2

  # for each ads
  links.each do |link|
    # go the details
    detail_page = agent.click(link)
    # capture name
    name = "there"
    element = detail_page.at("//div[@class='contact2Column' and @style='text-transform: capitalize']/b")
    if element != nil
      name = element.text.split.first
    end
    # go to send message
    message_link = detail_page.link_with(:class => 'btn btn-block btn-md btn-orange')
    message_page = agent.click(message_link)
    # replace name
    message_page.form_with(:name => 'msg_form') do |form|
      form.nachricht = "Hey #{name},

YOUR_MESSAGE"
    # send message
    end.submit
    log "Message to #{name} for #{get_id(link)} sent!"
    random_sleep 2, 10
  end
end

Make sure your replace the YOUR_ by your informations

Tried this again not so long ago and unfortunately it doesn't work anymore as it rely on very specific DOM elements, this should be a good base to create a functional one anyway.