Welcome to the thought process! Here's my cut/paste when this question is asked:
Things to check before buying...if you don't understand what some of these are, do a search here...they've been talked about a lot. :-)
1. Rotor magnets. If the PO hasn't specifically said he's done the jb weld fix, you'll have to do it. Good indication is if the ad says "new battery"; moved magnets mean the stator doesn't produce enough voltage, so the battery doesn't get charged fully. Also, if a voltmeter on the battery with the bike running at 5k rpm doesn't read between 14v-14.9v, then the bike has electrical issues that a new battery probably won't fix.
2. Stator. As above; if the voltage is low, it's also likely that you need a new stator.
3. Regulator / Rectifier. As above, if voltage is high, you may need a new r/r.
4. Chain and sprockets. Bad chain will cost you a motor. Don't ask why I know!
5. Lowering / raising links. If the bike is too short or too tall, changing the dogbones may be in your future.
6. Deceleration popping. Fix could be anything from an air filter cleaning to re-mapping the aftermarket power commander.
7. Suspension. If it's all original, it probably all needs to be replaced!
7a.Bad steering head bearings - lift the front tire off the ground and gently swing the handle bars past center a few times; if there seems to be a "detent" in the middle, you'll be needing new bearings.
8. Brakes. They're probably due for a flush. Just sayin'.
9. Clutch. If it chatters, it's due for a replacement basket.
10.Farkles - crash bars, skid plate, and luggage. Even if you don't think you need them, it's easier to sell them than to find and buy them!
2004 V-Strom 1000 (currently broken - parts received - repairs in progress - snag hit - repairs on pause)
Two Brothers with P1 tips, PCIII USB, SuperBrace Fork Brace, K&N filters, ROX bar risers, Kappa top and side cases, MadStad, Givi tall windshield, Givi engine guards, Sargent Seat, 12 volt air compressor, 530 Conversion, 17/44, SpeedoDRD