I've bought a white boxed garmin before, as I wanted a certain model that wasn't available new anymore (it had to be that model as it had a certain interface implementation that no others in the range had - and white boxed came in at £60 rather than the original £450 for that model).
Anyway, what I got wasn't a plain white box from the seller, but a printed white box from Garmin - i.e. the unit had been back to Garmin, Garmin had done whatever, and repacked it with brand new accessories/manuals/cables.
From memory the box had a blue stripe around the top with the Garmin logo, then printed details of the unit model etc. - a professional box from Garmin that just happens to be mainly white, without pretty product pictures, rather than a tatty box that the retailer found lying in the back somewhere. Imagine a "Tesco Value" style box.
Mine came with full warranty, registered OK as "new" on the Garmin website, including giving me the map updates that car units get for the first 30 days or so after purchase.
It looked absolutely new to me and has been perfect - but there is always the niggling doubt that the original owner had sent it back for an internmittent fault that hasn't been fixed.