Craigslist has boards and people reselling proformed shit. A lot of the second hand stores have some bottom of the barrel garbage. I would just say do whatever you feel like. It won't be that big of a deal if you buy new or used. Supporting a local snowboard shop is always a great idea, though.
Eldora is fine and will be fine for you. If you are going to drive up there you need snow tires, but that is the only reason you would need them. The front range area is flat.