I’d say it’s closer to $800 than $1500. Even if it is a great bike, it’s 7 years old, and it is only 105 (with ultegra RD). BTW, how much did you buy it for? If you bought it for a decent price (not some kind of killer deal from a friend or gift or something) that will give you a good idea of what it’s worth.
Also, 25 year frame warranties aren’t all that rare. Many big companies give lifetime frame warranties.
If you’re going to sell it, I’d try something like craigslist, and just start it high. If it doesn’t sell, just keep relisting it a little bit lower every time. As with anything else, it’s worth exactly what someone is willing to pay for it.