I agree, I'd ask less than $75. A 10 gallon tank is around $10 at Walmart, the tetra heater is around $12, the filter is only around $15, and the dividers couldn't have cost much, most use plastic canvas sheets from Walmart that are $2 for a 6 pack. So you're looking at roughly $40 worth of stuff, not including the sand, which isn't all that expensive. And for used, I agree, about $45 is the max I would ask for it. You'll be hard-pressed finding someone to pay $75.
Very nice looking set-up though.