You could consider going through an agency- one that offers the ability to (1) escrow money, (2) keep a written record of agreements, and (3) provide mediation. There are costs to this, but, at least until you trust each other, they may be the way to go.
They don't want that so bad that they will prepay me because they believe I'm not going to do that to them, they really want me to do their project, and so prepayment is a non-issue.
Edit: I haven't asked for $30,000 in advance before; more like 20 hour blocks of time, or thirds of projects paid in advance
In fact, maybe that's a better bellweather: look at samples of code. The more professional it looks, the more professional I can expect the interaction to be. Seems reasonable- what do you think?