My sister did the painting! Her paintings are original and absolutely fine, just not remotely interesting to collectors. Me paying a silly amount for it has no real effect on that. Similarly, a Picasso picked up for cheap from someone who didn't understand art is still worth millions because collectors care about the current and future desirability of Picasso originals, not whether the last buyer/seller was an idiot. And collectors obviously also pay a lot more in real terms for Picassos than anyone ever paid Picasso.
Or to look more closely at your example, the resale values of a mint condition Harry Potter first edition bought for £10 and a Harry Potter first edition bought for £30k are exactly the same and exactly the same collectors and auctioneers will deal with it, precisely because assets are identical and the market cares about how collectible the asset is rather than its individual historic price. (The market price of a first edition owned by Rowling herself is also the same, but she can change that pretty easily with a signature to make it more 'special' than other first editions)
Or to look more closely at your example, the resale values of a mint condition Harry Potter first edition bought for £10 and a Harry Potter first edition bought for £30k are exactly the same and exactly the same collectors and auctioneers will deal with it, precisely because assets are identical and the market cares about how collectible the asset is rather than its individual historic price. (The market price of a first edition owned by Rowling herself is also the same, but she can change that pretty easily with a signature to make it more 'special' than other first editions)