Honestly, almost all eBay sellers are kind of fishy. I suspect they all use shill bidding. If you want the week, I would proceed with caution. Get the estoppel letter before you send money so you can be sure that you are buying what they advertised.
I have been selling timeshares on eBay for over 13 years and have never shill bid. Why? It isn't worth the time, and most importantly if eBay catches you they will shut you down. I know this because years ago I had an employee that was let go, and was unhappy. He decided to bid crazy amounts on all of my auctions with his personal eBay account from his phone in the parking lot. I had over 50 auctions going at that time. The PIA part of all this is that eBay doesn't have an option to cancel bids on all auctions from one user, so he knew I would have to go in and cancel them individually. Unfortunately, that didn't happen. Instead, eBay's system flagged our auctions for shill bidding and cancelled them all. 72 hours later and too many hours on the phone with eBay all the auctions were back up, and lots of explaining was done to all the bidders who were previously bidding.
For someone who only lists a handful of higher end timeshares, then shill bidding might be worth it, but for me I simply want to sell the thing and move on. I did the numbers years ago and roughly 30% of high bidders on eBay do not follow through with their purchase. It is probably a bit lower now as less people are buying timeshares on the resale market. They either never respond or respond with their info to draft up a contract and then never sign and pay. My blocked bidder list on eBay is over 10,000 bidders.
Yes, there are plenty of fishy sellers out there who are not on the up and up. However, there are some of us who strive to have the most complete information as possible, respond as soon as we get a question, and will happily send any ownership documents or estoppels even while the auction is running.
Unfortunately, I can't know every little detail about every timeshare or club. When we don't put information on the listing that someone feels is vital, it isn't because we are trying to hide something. Most of the time that information isn't provided to us on the estoppel, or it isn't something that we know is important to the perspective bidders.
Buyers remorse does happen fairly often. It is cheaper for us to offer a second chance offer than relist it, but perhaps that seller isn't aware of that.
You would be surprised how many people I talk to on a weekly basis who don't understand how the bidding process works. They think that as soon as they put in $1,000 bid it will automatically go up to $1,000 with no other bidders.