I use a few free government APIs for the data (listed on the site, you can sign up for a key for free for all of them I think), plus a custom workflow I built that parses and ingests many other online auctions that states are mandated to make public, but which aren't part of any API or data pipeline I could find.
Expected load - about 1/10 of what it has seen today :) I had to scramble to make things more robust once this post became more popular, and am now looking at a few different options for hosting.
This was not really intended as a business - there is a very light "subscription" option right now, for users who want to create many alerts. I mainly wanted to try and fix this problem, share the solution, and get some feedback.