I'd try Craigslist first -- I've had great luck selling bikes on Craigslist, especially in the summer. Enthusiasts know what a Roubaix is so shouldn't be too hard in a big city like San Diego.

I'd try to get 50% to 60% of retail on a 4 year-old bike.

If you can't sell it on Craigslist, try eBay; bite the bullet and do a "no minimum" auction and it'll be gone in 10 days.