You will want to know for sure if it has a bad head gasket before you attempt to sell it. You can get test strips from the auto parts store. Dip the test strip in your coolant and it will detect whether or not any exhaust has made it into the cooling. If so, its a sign the head gasket has failed.

This will greatly impact the asking price of your Forester.

Currently a good condition 2003 Forester should get you between $3000-$5000 depending on mileage, body and interior condition.

If your forester has some bumps and bruises, needs brakes and leaks oil... chances are you will be lucky to get $2000-$2500

If it requires a head gasket replacement you might be lucky to get $1500-$2000 for it.