I don't know the salinity levels of marine tanks available off hand, but the tank itself offers such a small amount of surface area, I'd be willing to bet that just merely filling it with freshwater would effectively dilute any remaining salt to the point where it was unnoticeable.
Anyways, to be safe, a damp cloth wipe-down, or a hose-down and rinse outside.
I wouldn't worry too much about overflows and such, unless they are literally blocked by salt build-up, it likely won't matter when you fill the tank with freshwater.
Odds are, the previous owner of the tank drained it, and then put it into storage/up for sale (and even if they didn't, 98+% of the salt would be on the bottom of the tank).
If you see a whitish residue that you are concerned about, it's most likely carbonate buildup- also referred to as lime, scale, hardwater stains, etc. It may be unpleasant, but it's minor enough to not be concerned with at all. otherwise, the vinegar or razor techniques should work pretty well.