Okay, because no one seems to be answering the Venice question:

- They had a strong navy (and shipbuilding capacity), making a blockade difficult

- They traded with many nations, so no one group could cut off their food supply

- Fish

- They had a near monopoly on the trade of salt and spices, the former of which was important to everyone and the latter of which was important to aristocrats

(note: I read a few sources but this is not thorough research)