When collecting there's a selling price and a buying price. The difference can be -substantial-.
For example, stamps have a catalog. Literally a giant book, with all the stamps and variations, and the current value. (Ahem, buying value). Think if it as a giant "vendor neutral" price guide. [1]
Now obviously when you come to sell you mostly sell to a dealer. So you expect him to take some margin. He has to make a living. But the margin they expect will make your eyes water. (50% to 90% is common). Do test $100 stamp you have is say $10.
But eBay- sell direct. Sure you get more. But still a lot less than book value (partly because scams etc makes eBay risky for good stuff.) And it's it's lot of extra work. Useful for one or two pieces, less useful for 50. (And, of course, scams happen in both directions.)
That's if you know what you have. Selling someone else's collection on eBay when you don't know what you have is equally tricky.
In short, collections are expensive to acquire and hard to get rid of. The value in collecting is the joy of acquiring and having. Collections are (with rare exceptions) not a financial investment.
[1] my insight into stamp collecting died 20 years ago, so I'm guessing this catalog is online now.