You are right to be wary or eBay, but as the seller I can assure you I'm not trying to pull the wool over anyone's eyes. As stated earlier I picked it up for about £120 and spent at least that much again on a high quality frame, so think the starting price is fair.
I didn't commission the work, so can't provide a 100% guarantee of it's provenance but it is true to his style and his signature varied depending on what year he produced his work. I think it would also be fair to say that a gifted reproduction artist would probably turn their efforts to faking very expensive artworks rather than an Eric Day painting of a Herc!
I'm a serving Officer who despite my hopes never made it to Hercs, so just trying to rehome a nice bit of artwork, hopefully to some who spent time with the mighty Herc in the RAF!