T-bills work a bit differently. As of September 1997, the Treasury is able to debit a bank account for the amount of the purchase after the price is set by the auction, so there is no longer a need to send in a check for the full face value and wait for a refund. Treasury instruments can also be sold before maturity using this program for a nominal fee. Please see the article elsewhere in this FAQ for more information about using the Treasury Direct program. Of course treasuries can also be bought and sold through a bank or broker, but you will usually have to pay a fee or commission to do this, not to mention maintain an account.
Treasuries are negotiable.
If you own Treasuries you can sell them at any time and there is a ready market. The sale price depends on market interest rates. Since they are fully negotiable, you may also pledge them as collateral for loans. Treasury bills, notes, and bonds are the standard for safety. Interest on Treasuries is taxable by the Federal Government in the year paid.