The difference is intent. Overbooking intentionally creates conflict likely to result in a ticketed passenger unable to board. Overselling also results in a ticketed passenger unable to board, but was not intentional; at least not directly intentional. You could argue flying planes with different capabilities offers the possibility to have a lesser plane subsituted and that's an intentional choice, but...
Another way to think of it is what could an airline have reasonably done to avoid the situation? If it's overbooking, the reasonable thing is to not overbook. If it's overselling, they could choose not to fully book their scheduled equipment, but is that reasonable? They can't choose to have 100% reliable planes and crews and weather and ground operations. Stuff happens, and it's certainly reasonable to be upset when it does, but understanding why it happened can be helpful, so making a distinction between overbooking and overselling makes sense to me.
The airline sold you something that they can't deliver because they refuse to keep extra planes around. They refuse to keep extra planes around because that would eat into profits, and would mean their execs wouldn't be able to buy their third gold-plated yachts.
Another way to think of it is what could an airline have reasonably done to avoid the situation? If it's overbooking, the reasonable thing is to not overbook. If it's overselling, they could choose not to fully book their scheduled equipment, but is that reasonable? They can't choose to have 100% reliable planes and crews and weather and ground operations. Stuff happens, and it's certainly reasonable to be upset when it does, but understanding why it happened can be helpful, so making a distinction between overbooking and overselling makes sense to me.