Yes you can sell T-Bills before maturity through the OTC market based on the forces of demand and supply. For example a N100,000.00 face value TB maybe selling for less or more depending on the yield expectation of the buyers. If your face value is trading at a higher price, it means you can sell your treasury bills at a profit as such your N100,000.00 can sell for N101,000.00 or more. If your face value is trading at a lower price, it means you can sell your treasury bills at a loss as such your N100,000.00 can sell for N99,000.00 or less. Please note that the above analogy goes for the standard volume of N250million. However, some banks may buy off the lower amounts from their customers.