I won't address the sale of the old frame, other than to say your potential future buyer will have to be a kindred spirit, someone who wants a new platform for beloved old parts.

The entry-level frames are as good as the more-expensive counterparts in the line; what makes the price go up is the parts pick -- wheels and forks, brake types and derailleurs, mostly. For example, the Haro Flightline goes from just over $300 to almost $1100; I'd BET the frames are all pretty much the same.

My bud bought a basic Flightline for his wife, and the few times she did ride it before 'coming up baby', she had good experiences. (I just have to wonder who's gonna carry baby on rides, since he rides an old Epic!)