First I doubt you will find a legal version for that price. Ebay ones will probably be ones that people have installed, then copied the disk and are then selling the original disk while still using the software which is a violation of the EULA for the product.
Your question when posed in another context could be "A guy on a street corner offered me a BMW for $2000, so where else can I buy a legal one for that much?" 😀
Can you wait until Thanksgiving? I bought iWork 08 last year on Black Friday for $50 shipped, after corp discount and sale price. I think iLife was the same.
Er...... no! Apple has a policy of just 2 ownerships! But still, buying from ebay you're far better of buying a sealed box! Just bid on something ad you'll get it for somewhere around $55-60!