Allowing the spell alone to do this would make it kinda um, just a little to powerful as far as I'm concered.
The spell could allow you to look just like someone, but to convince them that you are the person your impersonating would take some acting, fast talking, or in this case the appropriate forgery skill.
You would also have to guess what that persons retinal scan, finger prints and voice look like to the computer system analyzing it.
To me that is like saying I cast physical mask on myself with 6 successes, so unless they can beat that they see my assault rifle as a violin, and me as the great Violinist "What's his name on the billboard outside" (Sorry short rant. Saw a player try this with a GM in a game once.)
The point is that this would give someone that they are impersonating an all to easy way to bypass any security measures in place without any additional effort on his part.

Well they are in a bad, bad spot if they don't have the skill required, Negotiation or Con in your example. However the "salesperson" still has to roll because a tie (no hits for either of them) goes to the defender (the rube). I personally would start dishing well about the given example penalties, the Social skills table is not given as an exhastive list, for something that over the top. Knowledge skills would likely start coming into play, as would the amount of cash the rube has, and what they have to do to come up with the money.

EDIT: In the Opposed Tests description it says they aren't suppose to have Threshholds. However for a number of Opposed Tests do the exact results created by a success do vary on the net hits. For example most spells targeted at living creatures. So as GM i would also interpret "success" on a sliding scale of net hits.

Also pertinent to your example is that cons are normally resisted by the group as a whole. So a good rule of thumb is don't send the rube out with your certified credsticks because he is likely to return with a bag of magic beans.

EDIT: JamesSL had mentioned he was, or planned to put together a much more extensive coverage of the Social skills. Even with 2+ pages plus a few other scattered rules don't really do the breath of the subject any sort of justice. You might pester him to eventually post what he has put together to tide over till Fanpro fleshes out the subject in a supplement book ala SR3 Athletics.

Interesting question. However i can't see the salesman succeeding in any meaningful way without rolling at least a single hit whether you call it an Open Test, an Unopposed Test, a Success Test, or Drunk Water Buffalo Test.

Don't forget you might actually want to buy some spells in there too...

I did consider that, but you can do much of what you can do with spells using spirits. You can survive quite happily with only a few spells to help you out when spirits just won't cut it.

A combat spell is always useful when you need someone down sooner than you can summon a spirit, command it, have it materialise, and thump something. It's always nice to have someone in the party with Heal, and a manipulation spell or realistic illusion is always nice.

Spirits with concealment can substitute for invisibility/stealth, spirits of man can substitute for many of the control manipulations, spirits with elemental attack can cover any elemental effects you need like zapping drones ETC.