You can also hit inventory and when you look at it you will see 3 tabs. - .It has all your enchantments (spells that you won or made), all your crafting.. stuff that you deliberately made in the MA, and summonings, buildings that you have won in events or produced in MA...
When you hit that button, you will see, on the bottom right "disenchant".. this will turn any of that stuff that you do not want into spell fragments..
For many of us, there are event buildings we win that we won't use, but selling them gets us nothing, turning them into spell fragments at least gets us something we might later use... the same with spells from tournaments, etc.
Hitting disenchant on the tab (page) shows how many fragments you will get from disenchanting.
Be careful if you have used displacement spells to move existing buildings into summonings to make space.. if you disenchant them, rather than put them back and then sell them, you will loose a lot.