If the phone is unlocked and for sale on ebay then there shouldn't be a problem using a former Verizon locked phone on ATT network. The only difference between the older Verizon phones and ATT is that the former had CDMA and GSM capabilities, wherease ATT is just GSM. CDMA is coming to an end at the end of the year.
If one buys a phone on eBay and it is locked to the iCloud one can only get it unlocked ethically if the person with the iCloud account initiates an unlock. If the phone is locked to a carrier, the carriers have criteria that need to be met in order for the phone to be unlocked. You can find those criteria at the carrier's websites.
If a phone has been reported stolen or if the previous owner has not paid for the phone completely or is behind in their account payments, then unlocking is a no-go.