Well they're not selling access to data, the data is freely available. They're selling a service that packages that data for you.
So either way sounds like a ToS violation. Which Twitter is usually pretty aggressive about shutting down.
*I was a founder of one of them.
Other than that, I would just make sure you position your service as something that benefits Twitter, describe it as something that allows you to create better tweets or engage with your followers better. The internet is filled with apps that sell Twitter data like Mention.com, SocialRank, and Social Animal.