Well... let's start out with some data points.
I think we can all agree that a used car is not worth as much as a new car. So the value is less than a new NSX. Right now, the dealer cost for a new NSX is around $80K, including destination charge. Acura is running a dealer incentive of $5K. So you can probably buy a brand new NSX-T for say somewhere around $80K, give or take a grand or two, which allows for $5K of dealer profit on the sale, which is fairly generous. Therefore, a used NSX-T (even one in showroom condition with very low miles) would be worth significantly less than that amount.
This 2004 NSX-T was bid up to $65K on eBay (didn't sell because it didn't meet reserve). I think prices tend to be slightly on the low side on eBay, due to the perception of high risk. Therefore, a used 2004 NSX-T would be worth slightly more than that.
So we know we're looking at somewhere between $65K and $78-82K. And not very close to the higher range.
If this were my car, I would look for $67-68K from a private party. I might list it at $72K obo if I had a lot of time, and drop the listed price to $69,995 if I didn't get a suitable offer quickly; I would list it at $69,995 if I wanted to try to sell it within a few weeks. Expect to negotiate downwards from your listed price by at least a couple large. Keep in mind that the market for almost-new used cars is exceedingly small, so don't expect a lot of serious interest.
As for trade-in value, why not ask your local Acura dealer what they would give you for it? Better yet, consider trading it in on your purchase of
the Porsche you're considering. In most states (with the notable exception of California), you will receive a sales tax benefit on the trade-in. So, for example, if your local sales tax is 8 percent, and they give you $60K on the trade-in, that means that you will pay $4,800 less in sales tax on your purchase - so that puts the same amount in your pocket as selling the car for $64,800.
Incidentally, if you are really concerned about the market value of your car, you might be better off getting a used car next time (no flame intended, just stating fact). The original owner of almost any new car takes a big hit in depreciation; in general, the older the car, the more it has already depreciated and the more value it will retain.