The color and the gas boiler make it not an original, at least from my 30 seconds of research. However, it could be modified. The running gear and bunker sure look like an Alisan. It doesn't carry the usual hallmarks of a fraud auction. The seller has 100% positive feedback, much of it recent as a seller, and has a track record of selling related items (mostly LGB stuff, but still large scale trains.)
If I were interested, I'd ask a few questions before I bid; maybe ask for some photos of odd details to prove the seller really has the item in his posession. Still, not an auction I'd run in fear from.