Look on ebay for the headset as you can probably get silly money for them now.
(prices incl postage, but that's gone up a bit now too, so add a few quid)
£40 was a good high price for the M901 a few years back and £20 for the M741 wa a good high price too
(I'd pay that now).
But if you want more, go for it. Check ebay sold and completed see what they fetch.
You can even search the sold prices in sale adverts on here.
Stem, I don't know, £20 maybe not sure. Probably for a specific project.
Dart HC, easily £20 now.