This is a Pain in the REAR!!!
When you are Editing your Equipment, You Need to make sure you Edit your "Bags" (it is the Far Left Tab at the bottom). There are "4 Sets" of bags you can have made up and you need to go through and Make Sure that the clubs you are trying to Sell have been REMOVED from ALL 4 Sets in the Bag.
I am the type that has 'One Set' of clubs, I play with the same Clubs ALL the time, there may be some players out there that have different sets of clubs for different courses, etc. (Must be Nice...)
WGT Please Look into this issue and consider a Better way of handling this.