I know this seller as I looked at an LS430 which he was selling a few months ago.
Despite what has been said about the seller by other members on here, I found the guy to be OK but then I haven't bought anything off him. He does seem to sell both Lexus LS vehicles and used LS parts too.
For £1k if the car stops, starts, goes fine, has sound mechanicals, good bodywork, good reliable electrics where everything works, and the mileage & history all OK then this could be a bargain of a car.
The advert says the timing belt has only been changed twice at 60k miles and 120k miles - it will be due another belt change at either 180k miles or 4 years after the 120k change was done.
Use your wits and common-sense for sure if you decide to check the car out. Make sure you try everything electrical in the car and that the paperwork all looks good.
You can also veriify the mileage & MOT history yourself if you ask the seller to provide you with some information on the car
Here's a link with more information
https://www.gov.uk/check-mot-history-vehicle
The most important piece of advice I would give to anyone buying a used LS is to check all of the above things are fine and in order - good LS400s are very rare and hard to come by.
I recently bought an LS430 to replace the LS400 which I sold last year after 13 glorious years of ownership.
Good luck