I've sold several musclecars, including two that I restored, a classic truck and also a couple vintage harleys on no reserve ebay auctions. Although I felt some of my vehicles, particularly the 2 restorations, were in line with the NADA description of what 'average retail' is, the actual selling price was closer to the 'low' book value on NADA classic car values. With this in mind, I consider the 'low retail' NADA classic values to be fairly accurate.
Here's a little blurb from the NADA website that I also feel is applicable to vehicles I've sold in the past:
Some of the vehicles in this publication could be considered "Daily Drivers" and are not valued as a classic vehicle. When determining a value for a daily driver, it is recommended that the subscriber use the low retail value.
The 'low' value vehicle represents what most of us would consider a fairly nice van, not a project or a parts van.
I think the 'average' values are probably what a dealer might be try to sell for and should be something that could do well in a car show with no additional work done. A very nice van might be closer to the "average retail" value. Very few members on this site own such vans...
I usually ignore the 'high retail' value because I feel it is not realistic unless you are watching the Barrett-Jackson auction on TV or something?
The 'low retail' value of a 1965 sportvan custom is listed as $3625, with a 10% deduction for a 6 cylinder engine, so consider $3263 as your van's NADA value.
The NADA value is a base estimate, and your vehicle's price can go up or down from there, depending on condition, upgrades etc.
Keep in mind that often, those custom non-factory 'upgrades' that owners do are considered unfortunate 'downgrades' by potential buyers and would result in a lower resale value...
I think the NADA value system is a little odd when it comes to our vans, since 1st gens were available only with the 6 cylinder, there was no v8 option. Also NADA lists a '66 as being worth a little less than a '65 or a '64, which I think is also odd... but maybe its because a '65 is older and considered more rare, hence being a little more valuable?