That's definitely an interesting issue. On the one hand, you're adding convenience to the shopper, but you're also "misrepresenting gameplay". Honestly though, I sort of doubt that anyone will notice that the gameplay is different than the screenshots, and that making the shopping experience easier for the user will be a beneficial trade-off.
I think you would just need to make sure you mention somewhere in the description that it plays only in landscape mode. Part of me also wonders if it wouldn't hurt to include at least one screenshot that somehow represents the landscape mode. Maybe more of a composite image that shows an iPhone in landscape mode or something. Not sure. I'd brush it off and say it isn't a big deal except I know there are people that look specifically for games they can play in portrait mode.
If you can't play in portrait mode don't include screenshots for this. Most people won't care, but if you get reviewed and someone makes a big deal about you misrepresenting it could really hurt you. You are just trying to do something because of your personal viewing.
