There is a community hub called Porthi Dre in Caernarfon who I think would appreciate them. There is no 2nd hand bookshop, but there are a number of charity shops who take 2nd hand books, so if there’s a particular charity you want to support, there’s a choice.
I don’t know Bangor so well, but I do know there are also a number of charity shops there.
If you want to offer them to forumites first, there’s more chance they’ll see the message in this thread - Second hand books up for sale / giveaway here! 📚