The one from Apple requires a $450 loan, in the form of a 2 year contract with a $450 contract termination fee. It is locked and cannot be used anywhere except the carrier and will be turned off if the contract is unpaid.
If you're finding one on eBay for $500 it will be of uncertain quality and origin. Likely it has one of these issues:
It may be severely used with cosmetic damage
It may be defective
It may have no warranty
It may be stolen
Typically it is safe to buy unlocked brand new condition phones on eBay for $600, but $500 indicates defects, flaws, stolen property etc.