As the original owner looking to sell, the best way to make sure is to check in with the phone’s original carrier (like calling or going into a Verizon or Sprint store) and ask if the phone is eligible to be activated to a new account. Other ways are as simple as notifying your carrier that you are switching service to a new phone or making sure that your account has no open balances.
For those looking to buy a phone from eBay you can use the IMEI or IMEID provided in the description of the listing (if you cannot find this information in the listing you can message the seller to add it otherwise it's a red flag) and call the related carrier if the phone is good to add to a new account.
In the event the phone has been reported lost or stolen it won’t have any chance of being activated unless it is returned to the owner or original carrier.
Check out our tutorial on bad ESN phones!