I bought a used iPod touch 5th gen on eBay, but it's connected to their iCloud!
If you have access to the iPod, go to Settings -> iCloud and then press Sign out. I think that should work without having to enter any password. Then you can add your own or do whatever you want. 
If you don't have access to your iPod because they're locking it using Find my iPhone, there are either two ways
1. They tell you the password to access the device
2. You force your iPod to enter DFU/Recovery mode so you can restore it using iTunes.
Hope this helps :)