Bought a used MB air- should I be worried about a root kit / keylogger? I ebayed a used Macbook. There is a new install of 10.8.3 on it. When I turned it on, it brought me through the user setup etc...

What's the chance that the os install is corrupted with a root kit / keylogger? How would I check for that?
I'm afraid there isn't a meaningful way we can tell you the probability that your system has a root kit and keylogger. The odds are certainly low, but not zero. Furthermore, if this is done well, it would be very difficult to detect.

One way could could check for this problem is if you use the computer to log into your bank account and then the money disappears and your identity gets stolen, perhaps it was infected. Obviously this is not the way you want to check.

While the odds are very low, if there is a keylogger installed, the results could be very very bad. If this Macbook is from mid-2010 or later (check your model for compatibility before proceeding!), you can use Disk Utility to erase the recovery partition, then hold down ⌘R while booting to boot in Internet Recovery mode. This will cause the computer to connect to Apple's servers and re-download the operating system fresh.