The 4-week bill is auctioned every Tuesday. Longer term bills, notes, and bonds are auctioned at other intervals. T-bills work a bit differently. Please see the article elsewhere in this FAQ for more information about using the TreasuryDirect 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.