Express Sell (available on iOS, Android, and on the web) and Trade are two different services we provide.
When using Express Sell (or 'Sell' for short), users are selling bitcoin directly to Bitaroo. We act as the buyer and set the rate, which is why Sell has no selling fees.
When using Trade, users are buying and selling bitcoin from each other. We only provide the platform and facilitate the transactions, which is why there are trading fees on top of the prices you see in the order book.
A good analogy to illustrate the difference between Sell and Trade is buying goods in a convenience shop vs in a supermarket.
Convenience shop (Sell): quicker process (you're in and out very quickly); easier to find what you're after; less products to choose from (making your options easier); higher prices.
Supermarket (Trade): bigger space with plenty of choices and options to choose from; could be overwhelming and complicated if all you want is one item, and you're not sure where it is; lower prices.