There is a destroy button, once they're on the map you can only destroy them.
Sell is only for "barricades" "cards"
And last thing is that you can only move the towers that you placed in this round, previous rounds towers get fixated. You can however get an upgrade card to fix that which allows you to drag any tower no matter which round it was placed it.
I think it was a lack of communication error and we've thought a better tutorial for it. So next version will probably have less issues like this.