What you need is a competent mechanic who has a lot of experience rebuilding engines. If you can find one with experience rebuilding NSX engines, so much the better.
Unfortunately, nowadays many dealer techs have a lot of experience replacing component assemblies, but not with rebuilding them. This is an "old time" skill that many younger mechanics may not have experience with.
In my case, I was fortunate because the dealer mechanic who has worked on my NSX for the past 20+ years has this kind of experience (he had rebuilt engines and trannies in other NSXs) and was able to diagnose whether my engine needed just the top end replaced or also the bottom end (turned out to be both). He was totally capable of rebuilding my engine, but replacing it with a used engine was the most cost-effective option (saved me $3-5K over rebuilding).
I'm not saying yours needs rebuilding or replacing, but I'm saying you need a mechanic who knows how to do both and can determine what options are feasible and the cost for each.
Oh, and if this were a car that I wanted to sell, at a minimum I would still take it to a qualified mechanic (with the rebuild experience noted above) to get a good idea of what those options are and how much they would cost. Then you can decide whether or not to get it fixed before selling. Otherwise you're probably going to have to discount the car by significantly more than it would cost to fix it, to take account of the uncertainty in doing so. I know this is probably not good news for you, but your car's value dropped by thousands of dollars when it overheated. At this point all you can do is examine your options, including fixing it before selling it as well as selling it as is. (Kudos to you for the full disclosure in your attempts to sell it.)
the potential buyer of the car will have a closer look if the repair has been done by a reputable shop or if it was a cheapo-fix. 
