First off I want to preface things by saying I do not own a Buell currently, but I am looking to purchase. Also I am not coming to you from the most technical of backgrounds but after working many years in the financial industry one thing i do know is due diligence. These are some common issues and fixes I've come across in my research.
Wiring Near Horn
Earlier XB's had potential short risk. Wiring to the horn could short out on the horn mounting bracket bolt. This was prevalent on bikes made before 2003.
: replace the bolt with a shorter one, or for maximum ease wrap the end of the bolt in electrical tape to prevent a short.
Many XB models suffer from sidestand shear. The bolt itself will break under stress causing the bike to fall over while parked.
: There was a factory recall for this issue but if yours somehow slipped through you could try an aftermarket sidestand or invest in some crash bars...
Front Engine Mounts
The rubber bolsters have a tendency to wear out, as the rubber wares it will induce frame vibration as engine and frame come into closer contact.
: Visually inspect these often enough. Replace the rubber as soon as you feel they have worn enough.
Buell changed the torque spec on the flywheel sprocket. Earlier XB's had been known to vibrate their sprockets loose causing menacing internal damage.
: Have a talk with your mechanic when in for service if you own a 2004 or earlier XB. You could do the fix yourself but make sure you use a threadlock instead of simply torquing the nut.
If you're like me and looking to own one of the last American Sportbikes don't let these little niggles deter you!
Good luck everyone, ride safe.