I thought this was a possible fraud, saying that it was an Aster. The color is something that i have not seen in aster shays, and i could not find that it look like the 3 Aster shays that i know were produced. But then again, you have more experience than I do. Good luck.
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.
As for buying on Ebay. I have bought a Roundhouse SRRL 24 and a Gordon Watson Argyle 4-4-0 ,which were excellent and great buys.
I also have an Aster Reno, that I bought as a kit.
As in any purchase you just have to be careful and not over pay for anything you get depending on the market.