They actually have two different products (well, more than two but anyway) the Venta Airwasher provides both humidification and also air purification. The basic unit (which is what I have) is just called the Venta Humidifier, and as the name suggests, all it does is humidify.
Personally, I might be ok with buying a Venta humidifier used, but I wouldn't purchase any brand of air purifier used. I have a Venta humidifier, so I know what it looks like and I can imagine being able to clean a used unit sufficiently. But I would worry about being able to fully clean an air purifier (even though, obviously, you would change the filter).
That's just me though.