In the broadest category on the site, the method matters more than any fact about one object.
Work out what carries the value
For nearly any collectible it is one of four things: edition, condition, completeness, or provenance. Decide which, and price on that.
Then ask the harder question: how certain am I that this is what I think it is? Certainty is part of worth. A piece you can identify is worth more than the same piece you cannot.
Why two of the same differ
One has numbering shown, a box photographed on every face and a certificate with a readable issuer. The other has a description. Describing collectibles.
How to find out rather than guess
Compare completed sales of the same edition in the same state. Match the state first — sealed, boxed, complete, displayed — because it moves the price more than the piece does.
Where nothing comparable exists, start low and let the room find it. That is not a failure of research; it is what a room is for. How auctions find a price.
"Rare" is not a price
It is a claim, and usually an untested one. Edition size is a fact; say that instead.
Questions people ask
Does a certificate raise the value?
As much as its issuer is worth. Show the issuer's name.
Is my childhood collection valuable?
Sometimes, usually as sorted lots. Condition and packaging decide it, and most things were played with.
What if I cannot identify something?
Describe it carefully and run it low. Somebody in the room often knows.