I was wondering if this software existed anywhere or in any form?
sorta like the PCpartpicker.com price graph?
Exactly, but the idea being it can look at any website and you just give it "clues" as to what to look for, then it does the rest and monitors those items for you. A good break down of all the categorizes of information to look for.
Well, there is this.
https://addons.mozilla.org/en-US/firefox/addon/the-camelizer-price-history-ch/
You probably wouldn't even need to do it graphically - as long as you can fetch a webpage, you could have a program sort through the code to find the price points in the code without having to do some sort of image searching, which apart from being bulky and slow could be less stable than the alternative.
That is, unless it does that already, but why call it 'graphical' when it clearly does none of the like?
So a GUI addon to a web scraper? Something could be knocked up in python (for ease) using the Scrapy library.
Also, I like your choice of http://www.usedlighting.com/ as a test bed ;) If you are on the fixture hunt check out http://www.solarisnetwork.com/ too.
EDIT: Also http://camelcamelcamel.com/ does this for Amazon products already, if you are buying something that can be bought from there. I used it combined with https://flubit.com/ to get an SSD for cheap.