I was wondering if there was a setting that would disable (make read-only or change to a label) the quantity text box on the checkout page. The markup can be found on the Cart.ascx control.
All the labels on the front end checkout can be changed by modifying the resx file associated with the page. Resx files are located in the \desktopnmodules\module name\app_localresources.
Also, in the buynow settings general cart setup section there is a checkbox to hide the quantity if you want to completely remove the qty on the cart page.