I would not take the word of a guy selling a tank like that.
It may or may not fit. The seller might be thinking of a wr250f.
I could be wrong, it might fit, but I've never seen anyone post about that kind of tank fitting on our bikes.
It might fit but require some custom fabrication or some stuff.
But I'd be very VERY cautious about buying that tank.
You could contact IMS and ask them if they think a tank for a wr450 will fit your bike.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs