If you could ever envision needing wear this armor while facing an armed threat, I would strongly advise against buying plates/soft armor you found on ebay or craigslist.
The thing about armor is that how it's been treated MATTERS. If the plates have been abused (dropped, stepped on, slid around in the trunk of a cruiser for a couple of months, etc.) they may be damaged. If they're damaged they may not stop what they're rated to stop. The only way to tell if they're damaged is to have them x-rayed.
This is less important but still a factor with soft armor. Extreme temperature fluctuations, direct sunlight, exposure to chemicals, etc. can all degrade the integrity of your soft armor to one extent or another.
Finally you have your carrier. The old adage "you get what you pay for" applies to an extent. Do you need to drop $2k on the latest and greatest from Crye Precision? No. Do you want to rely on a POS knock off vest made for Airsofters? No.
Your carrier is going to be holding up your loadout (magazines, med gear, commo, water, etc). It needs to be durable, threat appropriate, and fit properly.
It's your life your trying to protect. Don't go cheap.