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.

I know. Should I take it out or leave it in for full disclosure?