I have the answer for you; I don't know why the seller says it can be run in the "Le Mans Series" as it cannot, but I woudl guess he just thinks of "Le Mans Prototype" as a catch-all for sportsracers, or he simply knows nothing about what he has - which would be weird since the title correctly identifies the car.
That is not an F3 car - as the title notes, it's a Formula X car. Formula X was a failed attempt at a new mid-tier ladder championship in Europe in 2004(I think - might've been 2005). It died after only a single race weekend, IIRC.
The car was built by Van Diemen, at this point owned by Elan Motorsports Technologies. Elan took the chassis for Formula X and built a sports racer out of it - the Elan DP02 that you can occasionally see at SCCA events, and which runs in the IMSA Prototype Challenge.
Put simply, the car cannot be turned into anything that is, or ever was, Le Mans legal, but it can be turned into this:
