Currently only the 'Buy Now' button is displayed at the top of the page, while both the demo and buy buttons are additionally shown at the bottom of the page.  

I think especially for non-itch.io-users this causes the impression that buying is the only option available.  

This issue becomes worse with increasing length of the description.  

I suggest adding an option to also display the demo button at the top of the page (or just doing so by default).

I agree with this! 

For my 3D viewer Blaze3D this is especially bad. I embedded several videos and also listed links to all the models in use.

Someone not used to itch.io will never find the Demo button: https://martinbucksoftware.itch.io/blaze3d 

I added a note at the top of the page for this  reason.