Ask yourself this: why can't the founders afford to pay market salaries if they have successful businesses already, and had previous success with fundraising?

It's likely not that they can't pay you market wages, but rather that they don't want to for some reason. By that I mean they presumably could either pay out of pocket (if their other businesses have been financially successful), or they can sell the equity they are offering you to an investor, and then use the money they receive in return to pay you (that's the whole point of fundraising, after all!)

I would have an upfront conversation with them to find out the answers to these questions - maybe they strongly believe the equity is going to be worth a lot in the near future, and want you to share in that upside for your contributions. Or maybe they are trying to take advantage of you - only talking to them and reflecting on their responses will get you your answers.