I wouldn't buy a used pedal that I couldn't see/adjust in front of me.
I did buy 2 used dw 5000 single pedals those are pretty bullet proof. I just made sure all the adjustments worked and nothing was stripped when I checked them out. If your getting a used double also make sure the drive shaft doesn't have a ton of play or lag in it. Maybe try craigslist first? so you can see/play it in person if your worried about condition via ebay.