The prohibition is on using (and owning, I think), not holding wands. You can put wands in your Gringotts vault, ask an House Elf to bring you your wand, ... but goblins or elves aren't allowed to use wands. If they try to do it was a borrowed wand, I doubt it would go unnoticed for long. There is "priori incantatem" to check how a wand was last used for, and there may be global spells like the Trace that will warn the ministry if a magical creature uses wands.