When buying a used Verizon phone, be very careful.
You could use a directory and intermediary service like one of the ones you see when you Google for "used phone". These services may claim that they ensure the phone is clean before sale. However, it does not absolve you from needing to ask that the phone has a non-zero and clean (not reported stolen) IMEI and is able to be activated on the Verizon network without additional work. If the phone is rooted be very careful, as it might have an IMEI of 0, which will be a problem.
Note that if buying a pre-paid phone, it must have been used on the Verizon network for 6 months (paid at the higher pre-paid rate) before it can be used on a post-paid basis.
If you decide to go forward with the purchase, the intermediary service may charge the buyer a certain amount (e.g. $10) as part of the sale as well as charging you from the seller the remaining amount of the sale, such that on your Paypal bill you may see the bill for the $10 from the intermediary service and the charge from the seller.
You can track the device during shipping and when you receive it, you must first check the IMEI to ensure it is not 0, and then try activating it.
The seller may have unintentionally provided faulty equipment. A cheap adapter that may look like OEM may be poorly made and cause a fire. This is more common than you'd think. The phone might also have been damaged and cause the adapter to overheat when it is charging. You cannot determine this from pictures and the seller may not even be aware of the problem. Be careful!
Phones you buy used typically won't have a SIM card that you'll need for data. You'll have to get that from Verizon, and I'm not sure how much it costs or how that works, so call them first.
If you have any problem with your device don't mark the device as received in the intermediary service- deal with the intermediary service and the seller first and if needed go to Paypal to have them intervene, and worst case you would do a chargeback.
If you'd rather avoid the intermediary service, you could try buying the phone on eBay or Craiglist, etc. If you do that, take the same precautions asking the seller if the device has a clean, non-zero IMEI that can be used on the Verizon network (e.g. CDMA, 3G (old/slow) or 4G LTE).
Even though Verizon phones are CDMA, most newer ones use SIMs in addition for 3G/4G. SIMs can vary in size! You may need to go get a new SIM card from Verizon in order to activate your device.
Newer 4G LTE Verizon devices can automatically register on the network as part of the process, though you may have to reset your network settings on some devices if you are doing something like changing the phone number associated with the SIM.
Older devices will need to call *228 to register on the Verizon network. This involves having to enter "1" on the phone to register after it is answered. The old device must either be off or the battery or even the SIM removed if the phone cannot be turned off (if the screen is badly damaged).
To ensure your new device is both registered and able to use the Verizon network, you can call #832.
You likely will need to get assistance from Verizon in this process. Unfortunately for the hearing impaired or those using chat, the Verizon chat support cannot help to the same extent that the phone support staff can, and they are much slower. If possible, just call support instead of trying chat.
I do not work for Verizon, so don't ask me questions in the comments- just call Verizon for assistance.