A retail ebook is derived from an ebook sold by a publisher. The publisher's file might be encrypted or in a proprietary format (such as Amazon Kindle), in that case it is converted to an open format, generally ePub. Some conversions though mess up the formatting. Books converted from Scribd currently often have their formatting scrambled.
Non-retail means unofficial, produced by other means, usually by a scan and OCR of a printed book. These can be excellent or terrible quality, depending on the care the creator takes.
Many publishers make ebooks from their older books by this method and these ebooks, though retail, may be full of errors if they are careless.
Non fiction books with complex formatting and artwork or photos are often only available as PDF. A retail PDF is one sourced from the publisher's layout files. So the text in these is in the original fonts. You can also make a scan and compile that into a PD, often adding a layer of OCR text. However, the OCR is never perfect, and if you zoom in you will see the text is bitmapped, not smooth fonts.
And also, some uploaders state their files are "retail", but aren't. So caveat emptor.
I always check out an epub using Sigil, an epub editor. If I look at the stylesheet and see a bunch of meaningful names, and the styles used by a major publisher, then it's a retail ebook. You soon recognise the styles used by e.g. Random House. But self published books are more of a mess, made by amateurs. Still technically retail but I often spend an half hour or so cleaning them up to a standard that satisfies me.
