As she turned to leave, the flower retailer immediately replaced his smile with ...
I want to say he replaced his smile with his 'idle face' or his face without expression, but neither of those sound good.
The face that you pull when you're not pulling a face. Not a defined expression. Kinda like I've noticed I'm doing right now as I type (though this fact's no help to answerers).
The most common phrase would be 'blank', thus:
As she turned to leave, the flower seller immediately replaced his smile with a blank expression.
(No one says 'retailer' outside of industry magazines).
I like @ElendilTheTall's answer, but here are a few alternatives.
If you're trying to emphasize the transition from smiling to neutral, you could say something like
As she turned to leave, the smile faded from the florist's face.
If the smile was put on or phony, you could say
As she turned to leave, the florist immediately dropped his smile.
If you're trying to emphasize the new, inscrutable expression, one option is poker face, "an expression on your face that does not show your thoughts or feelings".
As she turned to leave, the florist immediately switched on his best poker face.
I have used a few times expressionless:
He had an expressionless face, what was he really thinking.
Lacking expression:
Their faces remained expressionless as they listened to the bad news.
instead of 'flower seller', I'd say florist--which is a flower seller.
blank expression can also be a staid expression
I would go for bland expression:
3 : showing no emotion, concern, etc. ▪ a bland expression/face ▪
Alternatives are stony expression and poker face. The latter, however, implies someone actively trying to block expression from their face, not necessarily the true absence of emotion.
You could use 'deadpan demeanour', but it does imply that the lack of expression is deliberate. Otherwise 'expressionless' is perhaps the best word.
My suggestion is the word inscrutable, although I realize an inscrutable expression could be characterized as mysterious, impenetrable, hard to read. Still, it's a good word if the person who is looking at the florist sees the expression but does not know how to interpret it.
What complicates things is that you give no reason why the flower seller lost his smile. What did the customer(?) do or say to make the smile disappear. Furthermore, why--if that is the case--is it significant for plot development?
Impassive, If you want to say it in a formal way.
Poker face would be the right noun, but I would say,
...the florist dropped his smile
or
...a vacant expression replaced the florist's smile
1) Poker-Face 2) Stoic 3) Straight face 4) Blank
The psychologist's expression for a lack of facial expression is "flat affect"
References: http://www.medicinenet.com/script/main/art.asp?articlekey=26293 & http://www.wisegeek.org/what-is-a-flat-affect.htm
