First, can you clarify whether it's a Retina II (as the title indicates) or a Retina IIa (as the text indicates)?

Next, is this the model with lever wind? If so, then it's a IIa. Prices on the IIa had been reasonably high several years ago (About $100), but have fallen as of late (about $50). However, I haven't looked at them in the past month or so, so it's possible they are back up again.

The flash attachment has almost no value. If the filters are Kodak-branded filters or Walz-branded filters, they have some value -- maybe $5-$10 each. If they are Series V, then they have minimal value (less than $5).

Next, if the camera has been sitting for decades, it likely would need to be serviced. While cosmetic condition tells a bit, what is equally important is whether the shutter fires at all speeds; whether the viewfinder is clear or cloudy, whether the camera focuses smoothly and whether the rangefinder is accurate.