Please take those as just pure speculation/talk:
Its really hard to find a buyer for items "rare" if there is not a specific collector markets...
You can try in some event/meetings/exposition - but probably you could just look online like social networks mostly/or if btc related post it here.
There was in the past similar offered to sell? Maybe you can start from there. You can try to reach directly them and propose your item.
I have no idea on specific seller / way to sell collectibles like NFT - plus here there are zero details ... your description doesn't add any clue.
You can try to have a look at scarce.city (aka try to submit your collectible to them - they have a better idea of a potential value imho).
My 2 cents in collectible (as Italian stamp collector...)
Most of the time if an user has something really valuable...he knows already where to sell it...

and how much could be the value.
Likewise a rare variation in stamp could me meaningless if there are no collectors/place to sell.
Or maybe there is really a "variation" of something valuable, but... the stamp has an impressive damage and become worthless.