I don't have a Gibbons catalog but it does not appear that they value the GB1 in NH condition at all, so the seller trying to place dollar amount on NH is, in itself, misleading. I base this from
https://www.stanleygibbons.com/blog...d-value-gum/ Gibbons website:
"The Great Britain Concise catalogue gives both mounted and unmounted prices for all stamps issued between 1887 and 1935"
"Gum
This would seem to be a relatively easy one – after all it says in the catalogue- 'The prices for unused stamps of Queen Victoria to King George V are for lightly hinged examples".
"Heavier hinging is acceptable for stamps issued before around 1890 but the majority of the gum should be clear and 'unblemished'. If the stamp has been mounted on a number of occasions or if there is a heavy hinge still attached, the price would drop to about half catalogue and from there on would decline fairly rapidly"