Defenders are still well over priced at the moment. eBay seems to be the worst, and I'd be surprised if any of them sell for what they are advertised for. £12k I personally think is lot for a 15 year old defender no matter how well maintained it is, but that's just the way it is at the moment while there are people willing to pay the money.
As
@dag019 says, service history can be fairly irrelevant, especially when you see the standard of work for some of these garages. We've even had garages charge for work they clearly never did, so 90% of work is done ourselves now. We do keep a record of the work but not all the receipts as it's more so we know what and when we have done stuff than for any future owners, which I don't think there will be any as we tend to run stuff into the ground.
I've never bought a defender off anyone with clean hands