I've done about a dozen rentals from TUG2.net / Redweek.com. Here's how it usually works: 1) I contact the owner by email and tell them I am interested and what week I'm looking for (if that isn't already posted, give them my contact information, and ask for their address, phone number, etc. and propose a time when I can call them. 2) They typically send a friendly email back with what I requested and include rental terms -- usually pretty standard ones: I pay them 1/2 up front, they reserve the unit in our name, and I pay the remaining balance 30 days before visiting. 3) We talk on the phone (This is nice. Most owners I've rented from are terriffic people and talking to them in person helps me feel I'm dealing with someone who is legit, competent, friendly, helpful, etc.) 4) I send 1/2 of the payment 5) They secure the unit in my name, give me the unit number, contact information for the resort, etc. 6) I typically call the resort to make sure all is in order, they have my name on the reservation, etc. 7) I get a reminder email about paying the balance from the owner, 8) I send in a check for the balance, 9) I get an email about the balance being received, 9) just before flying out I call the resort just to make sure all is in order, 10) I have a terrific vacation, and 11) I email the owner telling them what a great time I had and offer to be a reference for other renters.
Often, owners will give you names of previous renters you can contact if you want to do that too.
I've usually gotten owner rentals in the $800 - $1200 dollar range based on season, unit size, and location. (Better deals in Orlando than other places usually). I've rented in the Disney area (about five times), Cancun, Puerto Vallarta (twice), Victoria B.C., and West Palm Beach, FL.