Reads to me like he spent a wad of cash on the upgrades, doesn't want to\cbf certing it or returning it to stock, so he's try to sell it before the WOF expires and its illegal. Can't see a cert it would devalue it either, you can revert it back to stock and get the plate removed when selling it. It devalued because it has none not one.
The original struts\dampeners are likely to be tired at 150,000km which is why he likely fitted the Bilsteins in the first place, so fitting them would of waste of time\money.