Protip:
Don't sell your adventurers crap. If you sell them crap, they'll automatically use the crap, which means that, after all, they'll die faster in a dungeon, which means you won't get the items, which means you lose if you sell them crap. Give them good stuff for little money.
Also, food shops are good for the beginning, they give a lot of money easy and quick.
Then, if you want to get shittons of money in the dungeons, stay in one level and keep killing the same enemy. (for example only green blobs). If you keep killing them, you'll get a chain, more exp, and 2-5k item drops. Enemies respawn.