My contention is only this: selling coins does not lower the price.

You are introducing the added element "than bought." This changes the conversation from what a trade can do, to what is happening in the whole market (or whole order book).

I agree that a change in demand for a coin certainly will affect the price. I don't agree that it follows that a trade ("coins being sold") will necessarily lead to "than bought" (a decrease in demand across the whole order book).