They often command a premium price if they are in excellent condition because the vast majority of already-assembled vintage radios have seen much use, several owners and maybe even some dis-use (storage) and mis-use.
But the rigs commanding the highest prices are unbuilt Heathkit transceiver kits.
Heath made a lot of kits for a wide variety of DIY'ers - stereos, clocks, weather stations, ham accessories, etc. But it is their transceiver kits that bring in the $$$ for the sellers.
I've often wondered if those buying, say, an unbuilt HW-101 do so with the intention of building it or
|Unbuilt Small Wonder Labs' DSW|
Can you imagine finding any of these 50-year old rigs still new in the box, never having been used at all? Can't be done. Imagine what they might sell for! But their Heathkit equivalents are still out there, still unbuilt and turning up on eBay from time to time.
More modern examples of the value of no-longer-available transceiver kits that increase in value almost as soon as their availability ends are those kits from Dave Benson K1SWL (Small Wonder Labs SW+ series, DSW, etc) and Steve Weber KD1JV's ATS, MTR kits.
|Unbuilt Elecraft K2|
If history and trend analysis are consistent, it's inevitable.