Used Motorcycle Guide

So i'm going to buy a used motorcycle off craigslist in the coming weeks and i wanted to know what i need to look for (Year, millage, custom mods, maintenance history, etc.). I would also want to know your guys experience with used vehicles from craigslist if you had any. All answers are very much appreciated. Also any year/ millage that should be avoided. (older than 2005 or something along those lines.) (more than 30,000 miles no go?)
P.S. This is going to be my first motorcycle and i don't really want to spend more then 4000 on it and nothing bigger than 500cc.

I Moved it to off-topic so it will appear 2 times on the forums of SG