Sweet deal! I picked up four the other day for $30, around here they go for $40 or $45 usually. There's a guy right now selling in our area for $35 including new o rings, so that's not terrible. Craigslist is great and evil at the same time, cause when you see a deal on something you really know is below market value, it is too easy to pull the trigger. I believe I am up to 15 kegs. No, I do not need that many. But it makes it easier to do things like, when bringing homebrew to a party, I can grab an extra keg, and counter-fill from the keg that's been sitting in the keezer for weeks and weeks, no need to stir up the gunk. No need to take it there a day early so it can settle.
More kegs is better. More kegs, more fun!