Word for 'No Facial Expression'

Solution 1:

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).

Solution 2:

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.

Solution 3:

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.

Solution 4:

instead of 'flower seller', I'd say florist--which is a flower seller.

blank expression can also be a staid expression

Solution 5:

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.