vegetable/s stall

You could probably make a decent argument for either. I just prefer "Vegetable stall" cause it trips off the tongue more easily.
Vegetables stall is harder to say if you are a native English speaker, because you have this annoying "s" sound that must be repeated at the end of the first and the beginning of the second word. We call thaqt a "resounded sibilance". (Means that you wind up hissing like a snake).
Like the MAndarin word for wool fibre (phonetically DZZZHHH?) whenever I say it, it always comes out that Im asking for a little boy. ( )

Vegetable stall is correct. It is one stall and vegetable is an adjective describing what kind of stall it is. If there were more than one vegetable stall, you'd say vegetable stalls.

The convention is to use the singular.

vegetable stall
shoe rack
nail clipper
goody bag
coin purse
toy store

yes but the pural is of vegetables, not stalls. A stall can sell many kinds of vegetables , then , IMHO it can be a vegetables stall

Fm don't be silly, the aim is to be helpful here, and you were right first time.

It is vegetable stall. Car pool. Bike rack.

Many thanks, fellow members, for your help.

Ah, but then there's

notions department (very Edwardian, the idea, but there you are)
sports store (tho the Brits always seem to talk about "sport" rather than "sports") and sports equipment (even tho by its nature the equipment is used for only one sport)

My point was that there can be compelling arguments for either. Ive seen no evidence to counter that assertion.

Of course these things that seem so natural, and therefore the only way, are largely a matter of convention.

I'd say that these two are exceptions, and there may well be others. I believe they use the "plural" form because the "singular" form holds other meanings.

You can't buy a 'notion' from the 'notions' department.

A 'sport' is an agreeable person.

Key ring, car park, shoe rack, child care, tooth paste, log jam, plenty more.

Way more than 'plenty more'. By far, the vast majority, I'd say, McTag.