Selling items 'as seen' is fine as long as there's no specific legislation relating to the particular type of items in question.
For example, if a car dealer advertises a car as 'sold as seen' he's breaking the law because it's an illegal disclaimer. Similarly if you sell a portable electrical appliance (without a proper safety certificate) at a car boot sale, the fact that you put 'as seen' on it doesn't prevent you from being prosecuted for selling it without a PAT certificate.
However auctioneers frequently use 'AF' (= 'all faults', meaning 'as seen') in their catalogues for things like porcelain or fine art. By doing so they're actually helping potential purchasers, by flagging up a possible problem with the item .