To the OP, the prices you see online are simply asking prices.
Heck, if I purchased something at a great price and had the opportunity to list it (for free) at a much higher price, why wouldn't I do it? If some fish took the bait, I make money.
I have found that the lowest asking prices on Redweek pretty accurately reflects real sales prices (at least for Marriott).
Most online resources don't share what the actual sales price ended up being.
When it comes to timeshare, you should do a lot of research before buying anything. I have only purchased Marriott weeks. If you are interested in buying Marriott weeks, you need to decide the resort you want to purchase into, and then see what types of weeks are available (floating, fixed, annual usage, every other year usage, view type, number of bedrooms). Once you know what you want, the only thing that differentiates what you see online is the price.
So person A may be asking $10,000 for a 2BR Ocean View Annual usage week at Maui Ocean Club, while person B may be asking $20,000. There is NO DIFFERENCE between those weeks (assuming neither has any outstanding liens). Regardless of who you purchase from, you will have the same dues every year, and will have the same usage rights every year.
So why would person B be asking so much? Maybe he paid $60,000 for the week from Marriott and wants to minimize his losses. Chances are no one will buy it at that price. He also may not know the true market value of the week (which is set by buyers, not sellers).
Person A may be asking less because she wants to unload her week quickly. She did research and saw others sell recently for this price.
When I wanted to buy my Marriott weeks, I tracked resales every week on Redweek, Ebay, and TUG for half a year until I had an excellent feel for the true price. It saved me thousands, because I was willing to pay too much before I started researching. During that time, I rented units from owners and learned that owning (at least for me) is really just "discounted renting" because maintenance fees aren't a heck of a lot less than rent. Owning lets us get the weeks we want back to back, with no uncertainty.