I have a mate who has hoarded away many many many very hard to find parts that fit a later model Honda marque. He will never use these parts because his bikes are pristine originals but he has said to me he won't sell the parts either; ever. Hmmm!
I must say I have a somewhat different view. If these rare items are of little use to the owner then I think it is a good thing they are put out there for others to at least buy/bid on, thus they get the chance to finish their restoration. It may be that this is the only part they need to complete their project. Yes the person with the big bucks generally gets the worm and that's the way it is.
At the same time there is little reason for a seller to give that extremely rare part away. As you all know, we spend BIG bucks buying those hard to find items as nobody gives them to us. $600+ for a set of early carbs, $300+ for the 28 caps, $1000+ for a seat, $1500+ for NOS gauges and the list goes on. Yamiya charge a premium for their reproduction parts and yes they are good quality, but we still have to pay whether we like it or not and I have spent thousands over the years on parts. I recently bought a seat off a friend for not insubstantial dollars but I needed the part and the fact he sold it to me to to add another original item to my bike made my day, week, month, and I think he was happy to see an item reused as intended.
A 19 litre tank is probably the rarest or at least equal to the rarest of all early parts to find and they are a once a year offer on ebay and no reproductions on the horizon. KP