You might check the wind speed to see how many days, in your area, the wind was over 18mph last couple of years before you sell the Spider. You may be surprised at how many days per year the wind is that strong. It's about 50 days per year in my neck of the woods. That means that almost 1 out 6 times I want to fly the Spider would be my choice. As Murphy's law states, the time you really need it you won't have one. I live in the Chicago area, "Windy City" and all, but the reference actually regards the politics and not the weather. Chicago is about average as far as wind speed goes, and before I had a full vent I spent many days flying very uncomfortably or not at all.
If you can afford to keep it you might consider doing so. I recommend you have either a full- or extra-vent in your quiver.