I called my broker and also went on the Treasury Direct website. There is no commission to buy T-bills in the primary market, either broker of TD. Treasury Direct does not directly handle selling of T-bills before maturity - secondary market. You need a brokerage firm to sell before bills mature.
From the Treasury Direct Web Site - "To sell a bill you hold in TreasuryDirect or Legacy Treasury Direct, first transfer the bill to a bank, broker, or dealer, then ask the bank, broker, or dealer to sell the bill for you."
Given the above I am going to buy and sell via my brokerage account. FYI - Treasure bills count toward your account balance. Funds from selling T-bills are available the same day for trading. Not sure if this is instantaneous or with a lag. I think you would have to wait a day to transfer out of your brokerage account.
Cost of selling T-bills via brokerage in the seconday market is $1 per bill. They seem to only have $1,000 T- bills? Not sure why?