James, I think you may be mistaking Worldmark and Wyndham.
WorldMark owns two resorts on Maui, one on the Big Island, and one on Kauai. They are hugely popular among the membership, booking out immediately over a year in advance, and none are likely to be ever be made available for a rental on Costco.com. There isn't enough availability to offer rentals packages like that. Wyndham owns WorldMark, which may be where the name confusion comes in, but they operate as mostly separate companies.
WorldMark owns about 90 locations, mainly in the western part of the USA. It is a pure points system, without underlying deeds to any particular resort. You pay maintenance fees based on how many points (credits) you own. For those who like short stays, WM works well, but you need to be willing to stay where the WM resorts are located. You can exchange out to companies like RCI and Interval International (II) but that is an entirely different conversation.
In contrast, Wyndham owns everywhere, and availability is such that I think that is what you saw at Costco. Wyndham ownerships are at specific resorts, with underlying deeds that have maintenance fees based on owned unit sizes at that resort. The fees you pay can vary, depending on how that resort is operated. As a Wyndham owner, you can exchange to a lot of places, as well as with RCI and II, and there are resorts on all the Hawaiian Islands.
@Passepartout makes a good point: Hang around Tug and read more. Renting from an owner in different systems is a great way to learn what works, and what doesn't, for you and yours. Resort quality can vary widely, and owning in any system has its own merits for staying at other resorts in that same system.
There is no reason to rush, as resale timeshares are available every day of the week. There is no difference to you between "new" and "used" in timeshare ownership, because even if you bought a brand new resort from the Developer at big bucks pricing, by the time you get there, someone else has already slept in that bed. Resale is the only way to go. Bargain deals come and go, and if you miss out on one deal, another just like will be along shortly. As you have learned, it is easy to get into timeshare ownership, but it can be very difficult to get out of one. The eBay sales prices you are seeing are real (but be sure to check Completed Listings to see what that deal actually sold for.) I bought 12K WorldMark credits on eBay, with a fully loaded account (36K credits available to me once it transferred) on eBay for $1500, including all the transfer fees. That deal is really good, but I've seen better.
Good luck with your search. You're in the right place to learn plenty - Tug is filled with thousands of owners who have decades of experience, and no agenda. This is a fantastic learning site.
Dave