Is it legal to modify MIT licensed code and sell it?

Question

I have a project and I want to use a ready-made script that is licensed under MIT in it. But using this script separately will be redundant.

So I've decided to merge my code and the MIT licensed script in the same file. (Let's say I'll modify, improve and/or add new features to it.)

I'm planning to sell this work on a market, but is it fair (legally)?

NOTE: Meanwhile, I'll put (refer) the MIT licensed script's copyright already in the final file.