Can't reinstall OS X after erase

If you are trying to use Command and R to reinstall, that won't work. You need to reinstall Snow Leopard from your retail installer disk or from the original disks that came with your computer.

Boot from that disk in order to install Snow Leopard. Then update it to 10.6.8: Mac OS X 10.6.8 Update Combo v1.1. You can then re-download OS X from your Purchases page in the App Store

We can't begin to answer without first knowing exactly which model you have. What, if anything, did the seller give you other than the computer? In the future you should ask how to do something you don't know how to do before trying to do it yourself. Second, always make a backup first.

Well, I'm afraid that is what you will have to do. You can purchase the retail Snow Leopard DVD from the online Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99. You will be shipped a physical DVD disk. Boot from the DVD by inserting it into the DVD drive on your computer, then start the computer. Hold down the C key immediately after hearing the chime and wait until you see the Apple logo appear. You can release the key then. When the startup is finished just follow the onscreen instructions to install Snow Leopard onto your computer. Once you have Snow Leopard running be sure to update it using the Combo Updater in the previous post.

When you install 10.6.8 successfully you will be able to access the Mac App Store where you can download your own copy of El Capitan. You probably cannot re-download 10.10 because it belonged to the seller.

Yes, but you would need to clone the DVD to a flash drive, so you are in no better shape than you are now. Snow Leopard is not available on anything other than DVDs.