It’s not really the purchase price that’s the issue. There is a reason why those that can own a private jet buy (lease) new, it’s because only those who can afford to buy a new one can afford to actually run it.
The running costs doing get any cheaper on an older plane and you’ll not be doing any of the work yourself. You’d need a team of people so deal with all the requirements of operating a plane of that size and can travel internationally.
It costs £lol per year just to have parking space for it at a local airfield.
That’s why even the rich and famous don’t tend to ‘own’ their planes, they lease time on them from a company that operates a fleet and benefits from the scale that brings. They rely on the staff from that company to do all the administration, booking landing slots and air traffic routes, fly the planes, maintain the planes etc.
If you want a little single engined Cessna or something like that, sure that’s achievable but it would be so much easier and cheaper to fly first/business anywhere on an existing route and do the odd charter if that’s what you really want.
The same applies to super yachts, it’s only really those that can afford to buy new who can afford to run them and why they are effectively bespoke to each owner.
There really isn’t much of a market for a used super yacht, most just end up as charters for corporate events once the original owner is done with them (think Monaco GP etc).
Once they are no longer profitable on that circuit, they are scrapped. As pointed out above, if you can afford the cash burn of a super yacht you can afford a new one.
Someone who ‘only’ has £100m once can’t really afford the cash burn. The kinds of people who have these toys are on £50m+ per year absolute minimum.