You take a lot of risk when buying a locked device. It's a warning to you that the device might have been stolen.
If I buy a "locked" ipad will I be able to use it?
What type of lock?
-- locked to a cellular network. Could use it all you want with wifi. You would have to contact the cellular service to get them to unlock the device to let you use the device with another cellular service.
-- passcode, prevents access to your data. If the passcode is set & forgotten , you can reload from backup or wipe the device.
-- activation lock, prevents use of device if stolen. You need to establish proof of ownership for Apple to unlock. I doubt an ebay sale would qualify.
Unable to de-activate the activation lock means the device was stolen.