There are a few factors to consider when buying a used contactor. First, you need to determine what type of contactor you need, which can depend on the specifications of the electric motor you want to operate. You should check the data plate on your motor to determine the necessary current and voltage values.

Second, you should check whether the contactor you plan to buy is in working condition. You can use a multimeter or a testing device to test the contactor for functionality. You should also check for cracks, breaks, or other damage on the contactor body.

Third, you should check the source from which you plan to buy the contactor. Ask the seller for information about the contactor's history, and check if it comes with any warranty or guarantee. It's also recommended to buy from a reputable seller.

Finally, it's important to follow the instructions for proper installation and use of the contactor. Make sure the contactor is wired correctly and operated properly.

In summary, before buying a used contactor, determine the type you need, check for functionality, verify the source, and follow proper installation and use instructions.