Although I have not bought an Amtrak ticket on ebay, I would not hesitate buying one in a heartbeat. This is not to say I have the least bit of confidence in ebay (except as a source of amusement and appraisal), but I do have enormous faith in Amtrak and they, afterall, are the people auctioning the tickets.You will, of course, be able to get your Guest Rewards points if you pay for the ticket using your Guest Rewards credit card. And, unless there is some stipulation, you should be able to get double points since you are travelling by Amtrak.
The Amtrak deals are very good but so are the rail sale deals on the Amtrak web page, should you be lucky enough to get a ticket.
I recently scored a roundtrip ticket from Albuquerque to Los Angles for $50 on the Amtrak rail sale page ($25 each way) and that, too, was too good to be true. It was true however. But not nearly as good as my recent Albuquerque to Chicago round trip ticket which was absolutely FREE, thanks to 10,000 Guest Rewards Points that I recently cashed in on.