Q: bought a used ipod and it's locked what can i do?

Place the iOS device in Recovery Mode and then connect to your computer and restore via iTunes. The iPod will be erased.

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled

If recovery mode does not work try DFU mode.                         

How to put iPod touch / iPhone into DFU mode « Karthik's scribblings            

For how to restore:

Use iTunes to restore your iPhone, iPad, or iPod to factory settings

If problem what happens or does not happen and when in the instructions? When you successfully get the iPod in recovery mode and connect to computer iTunes should say it found an iPod in recovery mode.

Not that if it either a 5G or 6G iPod it likely has an activation lock

Find My iPhone Activation Lock

If that is the case yo will have to get the previous owner to remove the device from their account so you can activate/use the device

Turn off Find My iPhone Activation Lock