and the seller has had between 25 and 40 years worth of pleasure owning the weights they bought in the 70s-90s.
If you divide the cost by the number of years of pleasure then presumably it works out at about £1 a year for the pleasure of owning them. I think that's incredibly good value
They were a one time collector, they presumably had some way of working out what they thought the weights were worth.
And to them, what they sold them to you at is what they were worth to sell them on. Perhaps they just need to get rid/need the money and therefore the open market value is irrelevant to them.
Enjoy them, sell them, make the profit, whatever. That is the way of negotiation. No one would ever make a living otherwise. Everyone has to make their profit in every single employment aspect. Or no-one would work. Making a living is the point and everyone's skills have a value, hence a salary.
If you weren't going to make a decent profit on them at some point then presumably you would not have driven 200 miles to buy them. And they might never have been able to sell them at all.
I'd think that all the hours you've put in to gaining your knowledge plus all the books you've bought and read, have cost a fortune in time and money. And being able to buy paperweights at a decent price because you recognised what they are, is the reason you have spent that time and money in honing your knowledge. Expertise has value and costs money and time to gain.
It's probably about separating the heart and the head
m