TF2Glass.com
Unfortunately, since I wrote this site in php, I don't have a great way to publish a semi-functional version. Instead, here are some screenshots I took from back when it was running.
Home Page
A screenshot of the home page. It included a brief tutorial of how to use the site, announcements including the giveaways that I ran to draw attention to the site, and a top trading users section to display the most frequent users.
Interactive Trade Offers
This was the primary value of the site. This page allowed users to interactively select items from their inventories for the bot to purchase. Hovering over items would show the corresponding offer price from the bot. Clicking on an item would add it to the user's cart, either to sell if it was from the user's inventory or to buy if it was from the bot's inventory. Finally, once the user clicked "Send Trade Offer" on the confirm page, the bot would send a Steam trade offer to match the offer the user requested.
Database
I stored all state in a MySQL database. This screenshot shows a couple of my favorite queries from the database including recent offer states (as enumerated in the right-side table) and summary information. Notably, the "frombot" column shows whether the offer originated from a message to the bot through Steam, rather than the website. Before I created the website, the bot transacted with users with Steam messages instead.
Marketing
For marketing, I made posts on TF2Outpost.com and later Backpack.tf. As my bot grew in popularity and item quantities, I found making marketing posts quite repetitive. Unfortunately, neither site provided an official API to make posts (though backpack.tf did have one to query prices and later created one to support this officially). Instead, I reverse-engineered their listing workflow and determined how to replay similar queries to programatically make my posts. Further, since I could only make a limited number of posts, I chose the items most traded by users to include in my advertisements. This was my UI to generate and publish to TF2Outpost.com and Backpack.tf.
Steam Chat
Before I made my website, this chat message system was the primary avenue to trade with the bot. Pictured here is an example of using the bot to buy two items and sell another. The bot automatically added the currency (silver "refined metal") to balance the offer.
First Home Page
A screenshot of my first site for the bot. It didn't yet have the trade offer generation feature, but was popular for its prices table.
Flowchart
Lastly, here's a flowchart of the whole user experience. Sorry for the light-mode background!