Being 6'5, I can't say ice noticed eBay bikes in my size being cheap. I actually find then quite overpriced for the condition/quality of frame and components.
With that said, there is a common argument that small and larhe bikes are tough to sell and sell for lower prices than middle sized bikes because it's tough to sell them due to the limited market interested in buying them.
That could be true, but I often see the opposite, at least around me.
Large bikes don't seem any cheaper. And they sell just fine, assuming the price is reasonable.
The argument that prices are higher because the stock of available bikes is limited is quite intriguing, even though it isn't the popular argument.
54-58cm bikes are a dime a dozen and all over the place. So it would stand to reason that those sized bikes would sell for less since there is so much stock for each bike to compete against.
63-64cm bikes are very limited in quantity, so they could command a premium due to the scarcity.
Perhaps that theory doesn't play out, but I have found it does around here.