The site's functionality was restored as I figured out a way to bring it back and keep it running at a low cost. It will not receive any further updates or bug fixes.
My browser extension has the same functionality and much more, like: In-browser inspect, trade locks, doppler phases, bookmarking, floats, trade site bumping and much more.
It's called CSGO Trader - Steam Trading Enhancer, you can find out more about it here: csgotrader.app.

Changelog

Please feel free to report any bugs or suggest features.

2018-06-22

  • Inventory content is now loading separately from the page with a loading spinner improving user experience
  • Home page and navigation redesign
  • Added open graph image for social sharing
  • Miscellaneous style changes

2018-06-19

  • Even non-tradable and non-marketable items are listed like medals and default weapons
  • Commodity items like keys and cases that are the same type and unlock on the same day are now grouped

2018-06-17

  • All filters added and fully functional with search
  • New flat design for inventory items
  • Miscellaneous style changes

2018-06-14

  • Added filtering for nametagged and StatTrak items
  • Added Steam Profile and SteamRep Profile links to profiles
  • Started to redesign inventory items' list, they now show wall-to-wall
  • Miscellaneous style changes

2018-06-13

  • Merged inventory search and filter sections
  • Added filter reset button
  • Added slidebar to filter items by the length of the trade lock
  • Miscellaneous style changes

2018-06-12

  • Weapon filters are now listed vertically (thanks Heimburger for the idea)
  • Added tooltips so different icons are more distinguishable now
  • Fixed SG 553 icon (thanks PatkarR for the find)
  • Added favicon

2018-06-08

  • Improved item search and brought it to it's final form
  • Added filter icons to filter by weapon
  • Changed some of the styling on the inventory page

2018-06-07

  • Added inventory item search (still in beta)
  • Minor style changes

2018-06-06

  • Added permanent search field to the navigation bar for inventory lookups
  • Reorganized /iventory/ pages
  • Modified design and allocated places for search and filter functions on /inventory/ pages

2018-06-05

  • Inventories now load on their own pages instead of the homepage with the site's description
  • Added basic profile information and profile link to inventory pages

2018-06-03

  • Added Inspect links to items
  • Added Steam Community Market links to items
  • Added border coloring based on whether an item is tradable (green if it is, red if it is not)
  • Added item name tags
  • Colored items based on quality (red for covert, purple for classified, etc.)
  • Added changelog page