I would think defo a dealer with that amount of bikes. £500 looks like a pretty good price but then so much stuff is going so cheap at the moment compared to a year or two ago.
Thing with this Shorter bike is you would have got exactly the same position with a 700c front wheel and shorter headtube.
Went for £500, checked out his listing and he has loads of bikes for sale. I wonder if he us some sort of dealer?