At the $300 mark, I kind of look at it like this:
$150 = mini fridge
$150 = beer hardware including: CO2 Tank, regulator, Tap/Tower, drip tray, top rail, Sanke tap.
Other benefits to a kegerator include: ability to put 3 cornies inside instead of 2, wheels on cabinet, ability to put CO2 inside (even with 3 cornies).
Not saying you shouldn't build one, but if you have a hard time finding beer stuff on CL in your area, you're probably not going to save much going the DIY route.
Also, for watching CL, look at using the RSS feeds. It will auto-refresh your searches every few minutes. If you delete the entries in the feed that you aren't interested in, then only new entries will show up as it refreshes.