Go to the central market in the main housing zone (for example on Hib - Meath). Or maybe any market will do. There you will find a bunch of merchants scattered throughout, their name should tell you what they do, they have all of the stuff that you once had in your house vaults, and items and plat on your consignment merchant. I believe you can also get things like house deeds, porch deeds, vaults etc. from one of the merchants. All of this you can do without buying a house. Just talk to the merchant (for example the 'personal house vault 1 merchant', and a window will pop up, you can take things out of this 'vault' but you can't add anything.
However, I think if you leave everything there and then get a new house, the stuff might automatically transfer over. So if you buy a consignment merchant on a new house, it might already be setup with the stuff you had from your last house... and of course once you do this you won't be able to take stuff off of it in the central market. But if it doesn't transfer over then the stuff will still be in the central market. I think you might have to buy the house and put it under the same name as the character who used to own it. If you try putting it in another characters name then i'm not sure what will happen, you might have to manually transfer things over.
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