The biggest problem with aluminum shift knobs is that they are hotter than hell in the summer and cold enough to freeze the balls off a brass monkey in the winter! I had a wood knob on my MG that had an aluminum trim ring around the bottom and a steel nut below that locked it in place. In the middle of Chicago winters if you touched either with your pinky finger it felt like you were getting instant frostbite and it was incredibly hot in the summer (esp. if you have a ST with it baking in the sun all day).
Not trying to bash them, just a heads up to people dropping $137 on 'em before you get a bunch of irate D90 owners on your hands.
Good Luck with your sale, but I think you may be better off with these on Ebay.