Since you speak of bandwidth by monthly use instead of transfer rate per second, i take it that you understand as much of the subject as phone operators.
If you sell by anything other than slicing your pipe width, you're selling more than you have. Period.
I think you'll be hard pressed to find any ISP that doesn't oversell (at least to consumers - business connections are a different issue). In fact, it'd be extremely inefficient to have all that reserved bandwidth when only an extremely small slice is being used at any time, not to mention that the prices would probably go up ten fold or more.
Overselling is not a problem if the ISP manages it well (keeping a decent window to account for expect and some unexpected growth) and don't punish their users if they use what was promised.
If you sell by anything other than slicing your pipe width, you're selling more than you have. Period.