I have personally never ever traded with them. And I dont want to. But I know a couple of Gujarati folks that trade with them. And a couple of them that run it. The guys that run the business never really want to talk about in open so there is only so much that you can extract from them. But the guys that trade told me that its simply based on trust and once you commit its simply carved in stone. You have to honor your commitments no matter what.
Hop into any sub urban Mumbai local in the morning time and you will find many guys doing dabba trades
. This is how I learnt of it for the first time when I used to travel to work about a decade ago. I really do not know the intricate details of the business but a few basics will be explained to you by any Gujarati on a Mumbai Local if you are interested