If the seller won't send you these pictures, you may want to pass on making that purchase.
While it's tough to identify a counterfeit Coach bag, the serial number contains certain clues. According to Forbes, in fact, it can even be difficult for the company distinguish the real thing, although they have both overt and covert ways to tell (such as hidden stitch counts). Some are fairly high quality and look similar to their authentic counterparts, which makes it tough to identify the real thing. There's a huge market in fake designer handbags.