make sure the phone works and that you confirm with the seller the specification of what kind of sim card the phone will take and that they are buying the phone "AS... IS~"
most people just use the phone to talk on and want some kind of power source to recharge it when the battery is low, typical person
charge the phone all the way and let the phone sit for a while to see how long the battery will last, if it dies fast, don't bother selling the phone, you're just going to rip someone off. don't bother buying a new battery if this happens either, because I don't think cell phone batteries are cheap these days anyway
tell the seller the phone has been checked from several aspects and that it is in good working condition. sellin' stuff like this, people could put in some liquid damage on the phone and claim it was damaged upon receiving, I hate when people do sh*t like that, and they ask for a refund.
you'll probably get like 50 bucks for the phone if you want to sell it right away. don't lose sleep over it. sell it and get it over with, move on and never look back.