Endless Vacation Rentals is a part of Wyndham Worldwide. The Resort Vacation Certificates website is owned and operated by Resort Rental, LLC. which is an affiliate of RCI, LLC (RCI).
Skyauction is privately held company based in New York City. They negotiate travel, accommodation and other vacation offerings with hotels, cruise lines, resorts, brokers, etc. Some of the resorts I saw listed just happened to also be RCI affiliated resorts -- but I also saw II and some overseas exchange affiliated resorts as well.
Now having said that, some resorts have their own rental programs that are administered for their owners. This can be done by the resort management company itself, or through a broker or even a discounter; depending on each case. (Many resorts do this, and it is not unusual.) So that may be where some of the resort rentals you see in the Skyauction listing come from. However, I didn't see anything on the Skyauction website that led me to believe any of the exchange companies were doing this. But I suppose they could -- if if they wanted to compete against their own rental program or needed to get additional exposure.