Good questions.
I would imagine that the bug eyes only occur when the gene is doubled, which is why butterxbutter/lesserxlesser/lesserxbutter's have such a significant turn out... I'm not sure on the percentages, but no body really hears about mojavexmojave getting bug eyes.
I wonder if there are functional issues with the bug eyes as well.
I imagine that the ones up for sale are eating alight, so maybe it's just cosmetic? Either way, ball's don't really need eyesight to survive in captivity. Ben Siegel has an eyeless ball that eats and lives perfectly fine.
That's just my 2cents and opinion.