A quick look at
BikePedia shows the Optimo line as 2007, but came as a CAAD8 or CAAD9 models, with the 9's being higher priced.
The least expensive MSRP ~ $1,100, the highest ~ $2,500.
I would say an unridden 4 year old bike should draw about 2/3 of new, so yes, you may be underpriced. You need to determine which model you have.
Edit: may be a CAAD8 Optimo 1 triple ~$1350 MSRP