I can answer for Hyderabad - Chennai should not be too different.
2. Mostly in kgs
3. Most shops in Hyd have a fair one or two specific days of the week
4. Variable. Shops might dictate a price, but if you are a strong organic producer, you could negotiate something like 60-65% of retail price of the shop
5. Mostly delivery at the shop, some guys pick up. Pricing may vary 5% between the two
6. Very variable, assuming they do pay
Actually, one of my friend was creating a vegetable patch and we wanted to sell it. After several trial and error methods, at present we are selling in the nearby places around the farm itself at a fixed price.
Ours were purely organic vegetables but there was no premium from the buyers. The wholesalers were literally giving rates in single digits and on top of that the commission charges, transportation and loading & unloading charges were deducted giving us very little return. So, we decided to sell in the local itself.