My hosting company offers me more "stuff" than is really necessary, but I don`t feel like they are overselling. In my opinion the hosting marketplace is so flooded with new companies that everyone has to make ridiculous offers just in order to get business. I feel this is a good thing and a bad thing. Good becuase we get a great deal. Bad because I think the rate of failure for companies is going to be a bit high. In any situation I say make sure you keep a copy of any purchase agreements that you may have, and to also always have a back up of your files incase someting where to happen where a company is overextending there resources by selling packages that are too large and might suddenly be out of business.
If you are concerned about "overselling", you would stop using your cell phone, phone, cable internet, DSL....
Oh, ever noticed the local bus service always had empty buses? Well, don`t count on it because they certainly cannot handle if everyone in the city suddently decided to take the bus; actually you shouldn`t drive on the street because it certainly won`t handle if every residents in the city drive on the streets at the same time.
You should as well stop using electricity. If everyone turns on every electric items in their house, the whole system will shut down! Living in a cave will be safer.
All of those are overselling.
Overselling from a large hosting provider is hardly ever a problem. They have good statistics model to limit the overselling so it is never an issue. Once you get your site started, you will know that the bandwidth usage is extremely stable. A large hosing provider can oversell 100X and run a smooth operation.