Look, here's what the chart says for a '91:
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Under $30K:
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C Average condition and miles. Shows wear.
D Below average condition and/or high to very high miles. Needs some attention. Shows considerable wear.
$30-35K:
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A Excellent condition across the board. Very well maintained, low miles.
B Very good condition. Average to lower miles.
$35-40K
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A+ Perfect in every way. Like it just came out of a museum. Very low miles. Fully documented.
A Excellent condition across the board. Very well maintained, low miles.
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For a car to be in the B category, it would need to have average miles or better; average is about 5K/year, so a car with 70K miles is probably in the C category, which is why it's selling for $26K. Even more so if the finish isn't perfect and/or it hasn't had the timing belt changed and/or needs repairs, etc.
Also, there are a lot of other things that determine whether its condition and price according to this table. For example, A or A+ means that it is very well maintained, which means that all scheduled maintenance should have been done and there should be nothing that needs a repair. Most '91 NSXs don't fall into this category.