The fact that you only reveal the last hourly sales, which really has zero relevance to total sales and whether or not you "win", makes it feel like a game of 3-card monte, as I mentioned in another comment.

Maybe you need to be more transparent and show the total sales or the ranking. This might help show you're not trying to scam anyone. Then, it turns into something more where people can decide "do I want to buy this t-shirt in a 2-for-1 sale" since they get the next shirt free.