For the Hawaii part, it seems working OK. You send out a request and the owners response. As usual you need to ask very specifically about all the costs, rent, cleaning fee, key depost, if any, tax, etc
Deposit / Final Payment / Cancellation / Refund policies - you need to get it clearly stated by the owner.
I personally prefer owners who accept credit card the normal way (not thru PayPal). However, many owners only take personal check.
After a few e-mail exchanges, you get a feeling who are for real who are not and who are reasonable to deal with and who are not. So it is not just by price, you also have to judge whether you can trust the owner.
For our late May 5 day stay at Maui, the owner has a busy calendar (meaning many periods were already booked) and he took credit card the regular way for both deposit ($100) and for final payment (30 days before arrival).
I would look at each property's calendar to see if it is for real, i.e. are there many bookings ?