I'm planning on buy a used 2 ton car jack.
What are some warnings/items I should be on the look out for?
Is buying a used car jack a safe idea?
I'm assuming you are saying you are going to purchase a used "hydraulic" jack? If so, the main thing to look out for is if it is leaking or not. If it's a scissor style jack, ensure smooth operation. Test out the jack to ensure it works, such as does it smoothly jack your car up? Does it smoothly let your car down? Make sure YOU know how to use it. A used jack is like any other piece of equipment. If it is used and maintained correctly, it can last a lifetime.
As a safety item, remember to use wheel chocks when jacking your vehicle. Use jack stands to support your vehicle if you plan to go under the vehicle. A jack can fail a lot easier than a properly positioned jack stand, and as long as it is never over burdened (too much weight on it), a jack stand will continuously hold a vehicle up in the air without issue.
A trolley jack is used to raise the vehicle which involves a heavy weight. So trolley jacks should be maintained and serviced on a regular basis. Any damage that is caused during its use should be professional repaired, including the ram. A full history of the jack is necessary for an insurance company to cover any indemnity of its use in a garage. A new jack would not cost that much more than a used one in a good state of repair, and comes without the baggage of a 'used' jack. If you are offered a 'cheap' used jack you really have to ask yourself why.