Is there any way to recharge magic items (like swords, staves, etc.)apart from selling and buying them back? I guess maybe there is a high level spell called recharge? Can't remember if it's in W8. I thought Mana Stones would do it, but I guess not.

No. Mana Stones are for your Spell Points, and can be recharged if you don't use all 3 charges.
W-7 had a Recharge spell, you could make lots o'bux recharging everything up to 99...but not in W-8.

So is selling and buying back the only way? And does that always work?

It's not a big deal to me; I hardly ever use the invokable spells on items--mainly out of fear.

i don't think there's anything with charges that can't be recharged, and the weapons and jewellery all can be used down to 0 and remain, whereas the stones will vanish if you don't save the last charge.

I just remebered - the magic mirror (gadget) is also gone if you let the charges drop to 0 (but is rechargable normally)

mana stones, anyway. The stone of new life is an equippable stone (like jewelry) that persists after being brought to zero charge.