There is a large market where notes/bills/bonds are traded, so yes you can sell them later. However, if interest rates go up, the value of any bond that you want to sell goes down, because you now have to compete with what someone can get on a new issue, so you need to 'discount' the principal value of your bond in order for someone to want to buy it instead of a new bond that has a higher interest rate.
The reverse applies if interest rates fall (although it's hard to get much lower than they are now). So someone wanting to make money in bonds due to interest rate changes, generally wants to buy at higher interest rates, and then sell their bonds after rates have gone down. See my answer in this question for more detail Why does interest rate go up when bond price goes down?
To answer 'is that good' the answer depends on perspective:
- For someone wanting to buy a bond and get a return on their investment beyond the security that comes with T-notes/bills/bonds, it's not good at all. It means very low returns right now for risk adverse investors, in some cases not even keeping pace with inflation.
- For the Government, and taxpayers, it's good because the government can finance the national debt (either items coming due, or new debt) for almost free, and that means we are not taking an even larger hit out of the budget for finance charges on the debt. So for taxpayers and the Government that's been on a spending spree (with the brief exception of a few years during the clinton era) since Reagan was elected, the low rates are a good thing.
- If we step beyond Treasury issues, it's also good for Business that wants to finance new projects, capital improvements, etc, since it also means they can get money fairly cheaply via the corporate bond market.