Hi,
Thinking of 'a stall' to be in general some kind of place at a fair where one can buy goods which preposition goes with this word ?
Other common words for this are a 'booth' or a 'stand'.
at fair
__ stall
You'll hear people use all three.
At the stall ? Sounds like you are thinking of the stall as a place.
On the stall ? Sounds like you are thinking of the table at the stall, and the things that are 'on' the table.
In the stall ? I wouldn't say this, myself.
Best wishes, Clive