Abel Boyer, The Compleat French-master, For Ladies and Gentlemen, Tenth edition,
London: Printed for Samual Ballard, 1729
There are two particularly interesting phrases in this text. “Will you have them bound in sheeps, calves or Turkey leather” indicates that the bookseller was selling unbound sheets or books in a temporary binding. “This binding is not good.” suggests the purchaser was considering the purchase of a bound book. Together they seem to indicate there was not a set standard regarding the purchase of books bound or in sheets in early eighteenth century France. There is also a clear distinction between the items for sale by the stationer— paper, pens, etc.—and bookseller.
Peachey Conservation 
Another interpretation is that the bookseller had copies ready-bound in all three skins. This would certainly have been the case in eighteenth-century England for popular books; Stuart Bennett has shown convincingly that most English books were bound in small batches for the bookseller, and often offered in several forms of binding. The usual options included paper-covered bindings meant to be temporary, but standard price lists would also give the price of more elaborate treatments.
It’s interesting that “badly sewn” is one of the insults used by the buyer. How would he know? Bad sewing will show during the binding process or after significant use when the book breaks down prematurely, but it would have to be more ghastly than I can think to show in a freshly-bound book. One of my favorite gripes is against dealers and collectors who complain of books “too tightly sewn,” apparently because the book won’t open well; however, poor opening isn’t caused by tight sewing; and the ill effects of tight sewing show in other ways (in mmy experience, by a cockling of the spine margin and a distinctive clumping feel as blocks of pages are turned from right to left.) To me it looks like kicking the tires of a car, to pretend to knowledge you don’t have.
I think you may be right. It does seem like they are looking at the books. “There they be, as you desire them.” Or could he be showing samples?
So much of eighteenth century French binding is highly codified—books printed/ bound 50 years apart in different cities can look almost identical— it is some very muddy water.