I usually do timeshare rentals from sites like TUG2.net and Redweek.com. I haven't negotiated for Orlando, but that's because there have always been enough rentals out there for the week I want at a fair price. I get one of those and don't contact people who have nice units that seem overpriced to me, above market value.
I did, though, one time negotiate for a Marriott timeshare rental in West Palm Beach. I needed two "two bedroom units" that slept 8 for our spring break week. I booked one right away from someone who indicated they were interesting in trading on TUG2.net. They were going to trade their unit for a cruise discounts though Marriott, but rented to me instead, for what I thought was a fair price, $1200 for the week. Most other places aren't as overbuilt as Orlando and usually prices are higher. // Retail these were $450 a night ($480 if taxi is included) I then on TUG2 also saw someone else with a for rent price for the same resort for my week. Crazy, though, they were asking $3000 for the week, way more than I would pay for an owner rental. I emailed them and told them that I had another rental for the same week for $1200 and would love to rent from them, but for that same price, which is in the range that I can typically get an owner rental for. They said, no way and that they could easily rent it for way more. I kept things friendly and said contact me if you change your mind, as I'm really interested if they would be willing to come down in price. This was 11 months before my spring break. In the meantime, I booked a hotel room at an adjacent property (not anywhere near as nice as the condo -- something, though at the $200 dollar price point retail) that I could cancel and kept looking, but didn't see anything else at a reasonable price for my week. And what do you know, the Marriott timeshare owners contacted me 10 months later, one month before my spring break and told me they would rent to me for $1200 for the week if I was still interested. I grabbed it and canceled the hotel room. Anyway I was thrilled to have two units at a good price here.
I also one time in Cancun couldn't find a reasonable rental for a week at a timeshare I wanted to rent from, so looked for people who had my week who were trying to sell their units for really high prices that they could never get and I knew they wouldn't be able to sell. Anyway I contacted a guy about three months before my spring break about renting and he rented to me for $1200 for the week, the same price he would get from the complex if they rented it for him. From me it was a sure thing and he thought I seemed really trustworthy. From them, he'd only get the money if they were successful in renting it out. I saw a bunch of overpriced sale units but picked to contact him, as he had the perfect ocean front location vs. others who were in side building (got a map of the resort online.)