no matter which side of the sale you're on, the person on the other side could be looking to rip you off.
Sage words.
My stage piano was purchased pre-owned, and I bought it off Gumtree (Australian on-line buy/sell intermediary). I've never had a single bit of trouble with it.
What I did do first though was audition the keyboard. Before I parted with my cash I tried every key, the damper function, every control and inspected the keyboard itself for wear and damage. I'm not sure I would buy a used keyboard if I weren't able to do the above.
Slightly away from the original question, but a good cautionary tale below:
In reference to Michael's post - I also sold my old stage piano via Gumtree and that experience was a living nightmare. In future I will only sell to buyers who meet me personally - so they can test the keyboard for themselves and we can agree that it works to their satisfaction.
Although most of my keyboards are gigged heavily, I keep them in immaculate condition. They all have hard cases when they travel and I've never put a dent, scratch or mark on one. They have dust covers and they all work 100%, otherwise they're no good to me. The 'board I sold was no exception. The reason for the sale was playing larger venues (so more space to lug bigger 'boards) and wanting to upgrade from a semi-weighted 76 to a weighted 88.
The guy gets the keyboard. First text I get is about how it doesn't do something or other. I politely text back explaining that it's a stage piano not a synth, and referring the purchaser to a website where he can download for free a type of sound that might help him. Then I get a note about how the keyboard doesn't sound very good. I again politely text back saying it sounds how it sounds, and these things are subjective. Personally, I thought it sounded wonderful and gave him some links of videos he could watch of the 'board (the actual one he had) going through its paces in front of an appreciative audience.
A few days later, another note about how it's making a strange static noise. By this point I've had enough and I said that if he wanted to send me a recording highlighting all these deficiencies, AFTER he'd checked his cables, DI, PA and speakers, I'd happily refund his purchase price (less postage), just so I could stop putting up with his harassment.
Didn't hear from him again.