As a purchaser, when buying a Single Jersey Circular Knitting Machine, the key considerations should include the quality of the machine, the features and capabilities of the machine, the price of the machine, and the availability of spare parts. 

The quality of the machine is important because it will determine the longevity and reliability of the machine. The features and capabilities of the machine should be considered to ensure that the machine is suitable for the intended purpose. The price of the machine should be taken into account to ensure that it is within budget. Finally, the availability of spare parts is important to ensure that the machine can be serviced and maintained over time.