Droid application legality question

Hey guys, I'm not sure if this is the right place to post this but I had a question. I was wondering if it is legal to create and sell an app in the Market that brings the functions of another site (that I don't own) to an app format.

For example, let's say people use site "A123" for some tools that the website offers. People pay to use this website and it's tools.

Would it be legal for me to create and sell an app that brings the same functionality to the users phone? They would still be using their same paid account (that they use to visit the site on a computer). The only money made would be selling the app of a website that's not mine.

I hope this makes sense. Thanks in advance!