If you desire to purchase a new resale, as you mentioned, I would recommend a Marriott.
Not sure which Vistana resort or season you own, but what resort location you're able to trade into
is a direct function of what you offer in that trade.
A low-demand resort or season will not pull a higher rated resort or season as often as it used to,
so "buy the best you can" in your next purchase. Deals are out there if you're patient.
Ebay has been my go-to site for 8 of 11 resale purchases, and I've had zero problems.
As an example, I bought a Marriott Platinum Manor Club Sequel lockoff about 2 years ago for $800,
and it has performed way beyond what I imagined. It pulls other Marriott platinum weeks,
and (if I choose) splits into two weeks, each pulling another platinum or gold week.
So far I've not failed to get the exact trades I wanted.....
(one to Aruba OC, one to Maui OC, one to Grande Ocean/Hilton Head, & one to Lakeshore Reserve/Orlando).