Well, assuming you are talking Capri 22 sails, may I ask what price you are looking to get for your sails? In my experience, used sails, regardless of age, rarely fetch more than 40% of the cost of new. I know that's not a "fair" price for a sail that's been used 3 times, but you also must take into account that when it comes to used sails, your definition of "practically new" and someone else's may be far different.
Serious racers are likely looking for mylar tri-radials or even string sails and aren't interested in Dacron. If you have a Dacron headsail from a premium loft (i.e. Ullman) you might have $1200 in a Dacron sail. When you look at what some of the off-shore suppliers (like Precision or FX Sails) will likely sell you one for $800. Suddenly, $600 doesn't look like a real bargain for a used sail. You must take this into account.
You may find a wider audience for your sails on eBay. I have had great luck selling sails there. I am often surprised by how much sails go for in an auction. Good luck selling!