@verysamish Thank you! It has been pretty easy to weed out the people who aren't serious. In my first message I give them a ballpark of my price range, which is in the thousands. Most drop off there so I'd rather not waste a lot of time on someone who wants to pay $300 for a book. With the few who are still interested I've set up a phone call or zoom call to discuss it with them, so I can get a good idea of who this person is, and also ask for the manuscript before we talk so I can decide if it's even something I want to illustrate. I have a good sense of people and only work with people who aren't showing any warning signs of being sketchy or flaky.
I get the first third of the money before I start anything at all, the second payment before starting the final art (after sketches), and the final payment after I send them low-res images of the final art. They don't get any of the high res files until after I'm paid in full.
One more thing that has been important to me is to only work with people who love MY work and want to give me creative control to do what I do, and aren't just looking for any illustrator. This has been key to a happy experience, I think! And I charge separately for revisions and have it in the contract that I have ultimate creative control, so it's avoided the endless revision trap.