i am not sure if there is a market for donkeys. Only few people use it for carrying weights and probably they are breeding the donkeys. About the donkey meat, No idea… 
Reg the Milk market, i saw some people selling donkey milk in cities, especially for new borns and infants, as the mother’s milk sometimes doesn’t give enough strength needed for the kids. This milk is sold at a fancy price of 60 Rs/L and people were ready to buy it as its something related to the health of kids, and the infants would consume around 250 ML to 500 ML per day max. If you want to market donkey milk, you should be targeting these consumers.
Do a thorough survey before stepping in, as I haven’t seen anybody who has targeted the demand for donkey meat and milk. 