It is both safe and common to purchase used farm equipment, including tractors, as long as the buyer knows what to look for in regards to a reliable piece of used equipment. Inspect he tires, frame, cab, engine compartment and engine thoroughly before making the purchase.
If the tractor has a hydraulic system, have it tested for leaks, pressure loss and fluidity of motion. Additionally, if the tractor is going to be used with equipment such as augers, grain carts or spreaders it requires a functioning power take-off shaft. Inspect the shaft for functionality and damage and inquire about the horsepower driving the shaft to ensure that it meets the minimum requirements for any equipment or attachments that the tractor will be pulling.