Have you purchased the motorcycle yet? If you haven't put pen to paper, there are a few things that I'd recommend checking for before you actually purchase it.
Get a BUELLtooth and check any stored Check Engine Light codes, plus the AFV that you find.
The AFV is Adjusted Fuel Value. It's a fudge factor built into the system that allows the ECU to easily make changes to the air/fuel ratio based on a variety of different factors that the computer processes, including the input air temperature, head temperature, and the detected mix by the O2 sensor on the exhaust after it finishes a cycle. This value, though a tiny bit crude, is the single most important tool we have to determine what the mix inside the cylinder is doing.
The AFV is intended to be set at 100.
When the AFV increases, it means that there is a lean condition and the ECU is compensating by adding more fuel to the mix. This is because the cylinders are either getting too much air (leaking air intake seals are a common problem on this bike), or the fueling system is not delivering enough fuel (also tends to be a common occurrence on this bike). If your AFV is under 110-115, I wouldn't worry too much, though there is a minor issue to be addressed. If it is in the 140+ range, it means there is a serious (though very, very solvable) problem and you should make 100% sure you know what you're doing when you put money down.
When the AFV decreases, it means that there is a rich condition and the ECU is compensating by pumping less fuel to the mix. This could be because of a leaky injector or an air blockage.
I bought a bike a while ago and wish I would have done those things, as the bike ran great, or so I thought, and when I started to dig into it and have problems, I discovered a lot going on that I wasn't prepared for.
That being said, if you find a great example of an XB12s, you will be addicted beyond hope. They're fun, nimble, and the power delivery they have is perfect for being around town. Plus, they look and sound so, so badass. Just a little bit of warning, though - you're going to have to answer the question "What is that bike?!" many many times. Enjoy it!
Welcome to the club, man!