I've accidentally sold a couple moves I didn't want to sell. Thing is, I know for a fact that I wasn't paying attention. The only times this has happened to me was when I was in a mass selling spree and kind of on auto-pilot.
However, the last time this happened to me was many months ago, before the mass-sell feature was added. Since then, I've not accidentally sold a move I didn't want to sell.
Point is, pay attention and this won't happen. The confirmation screen you get after pressing the sell button is enough. If you aren't paying attention to what you're doing, then chances are no number of "Are you sure?" screens will prevent you from making a mistake.