I just had a thought regarding just "who" would pay a 20K+ price for a low mileage, clean rust-free 7.3:
A guy who has previously owned a 7.3 and for whatever reason doesn't have it anymore (theft, totaled, wife took it in a divorce, etc) but still NEEDS the capability of a 7.3.
Another candidate would be a previous 7.3 owner that realized the potential, but ended up with a $7,000 POS that looks great on 35's, but leaked, shook, broke hubs, wouldn't start, trans went out, had rust problems, etc. and he said f-it and got rid of the POS. Rather than take over someone else's nightmare again, he's now looking for an un-molested original to avoid most of the issues with one that's been beat, modded and lifted. They realize the value and rarity of a low mileage original truck.
These types of buyers are out there. They will pay high dollar because they intend to keep it a long time, perhaps indefinitely. You just have to get the truck enough exposure that you will find one of these guys.