If you're looking for a used iPhone, find one in good condition and for a reasonable price, but the catch is, Activation Lock is on, it's an immediate red flag. An iPhone that has been Activation Locked can only be unlocked with the iCloud credentials of its owner. Therefore, buying an iCloud locked phone is equivalent to buying a very expensive paperweight.
Despite what YouTube videos or websites say about hacks or programs that let you break past an iCloud lock, you can't do it. It's a layer of security designed by Apple to protect people from having their phones stolen and then resold by thieves. As such, you can't get around it unless you were the original owner of the phone who put the Activation Lock in place.
Not all iCloud locked phones are stolen, of course. Occasionally a seller might forget to deactivate their iCloud account through Find My iPhone before handing it over. That's why you need to check and make absolutely sure that step is done before you take possession of a new iPhone.
If you're buying off eBay or online, and you can't verify the iPhone is unlocked, don't buy it. If you're buying off of Craigslist or in person, and the seller is unable to unlock it for you before handing it over, do not make the deal.
There are a lot of great deals on used iPhones out there. Activation Locked iPhones aren't one of them.