Stalk market for sure, I made 10 million the other day thanks to a lovely friend of mine who had high prices and left their gates open for me, first time I'd really spent a lot on turnips as well.
Also late night bug and shark hunting on the island. I'm told bell boom really helps boost it as well although I do it without.
i gave away plenty of marshals, julians, and ankha, merengue ALONG with selling them. However, people who i gave away villagers to ended up just selling them back onto the market.