13" MacBook Pro Retina back cover won't come off?

I purchased this MacBook Pro Retina on eBay. When I got it, I noticed a screw in the lower left corner is sticking out. So I went online and bought a screwdriver to tighten it up. When I got the screwdriver instead of just tightening the screw down I decided to unscrew all the screws and check out how the inside of the laptop looks. Well after I unscrew all the screws, the cover will not come off. It comes loose on the top where the hinges are but something is holding it down on the bottom? Also this laptop is 8 months old and it only has 19 battery cycles? Maybe the seller replace the battery and they messed something up? What should I do?