They are always selling the certificates.
http://www.skyauction.com/action/offer/2768903
If you look at that page, it has the link to their inventory. I wouldn't buy a certificate until I see something in the system I want. You can search the availability from there. Once you do the search it shows the resorts and if that week just costs the certificate or the certificate plus some money, ie. certificate plus $100. The busier weeks will have the additional money added beyond the cost of the certificate.
They tend to have Sliver Lake Resort, Cypress Palms Resort and Cypress Pointe which are all good resorts.
I've seen people recommend Orbit One as well.
Then they have deals like this one below. 7 nights in a 3 bedroom at Cypress Pointe Resort. With specific dates it's available. With this one you need to make sure your dates aren't in the Surcharge range of dates because then I don't consider it a good deal at all.
- Date Surcharge (5/5/2017-5/20/2017, 5/26/2017-5/28/2017, 7/1/2017-8/19/2017) for $300 per room
May 6, 20, 21, 27, 2017
July 8, 15, 22, 29, 2017
August 5, 12, 19, 20, 26, 2017
September 1, 9, 16, 23, 2017
October 1, 20, 27, 2017
November 11, 24, 25, 2017
$1 bid
$299 taxes/service charge
$20 processing fee
So, $320 for a week in a 3 bedroom condo.
http://www.skyauction.com/action/search?otid=150990