Agree that if you're a minority shareholder in a company and have a bad relationship, you can expect to get abused. You need to make sure that the share purchase agreement is sufficiently thorough to foreclose the possibility of disproportionate dilution or other tricks.

Unfortunately, the cost of hiring a lawyer to do this is probably several hundred dollars (at least), which may well not be worth it or feasible for you right now. If they're in an accelerator, they may have the cash to pay you. Sure, you'll kick yourself if they end up billionaires someday, but you can be certain that they'd have found a way to screw you if you'd taken equity instead.