A potential buyer wanting to know what your traffic "was" or "once was" isn't something that should necessarily come up. I'd tell buyers your traffic is 6,000 uniques, and leave it at that. If they ask about the past use the 75K number then (and only to highlight the potential of the site, not what it 'once was'). The way it's phrased now looks like the site is heading south.
A buyer would like to know that, with some media events, you can drive a spike of 75k visitors. A buyer might have some new ideas about how to keep them coming back.
Also, this is not good: Our traffic has hit up to 75,500 monthly visitors, averages 40,000 but currently is at 6,000.