That would be Seth Nock. And yes, he's gotten good reviews on this forum from people who have bought from him.
Your cheapest way into HGVC would probably not be to buy at Sanibel, but to buy elsewhere (Las Vegas is frequently cheap), and use your points to get reservations at Sanibel. However, I understand that the Florida affiliates can be difficult trades, in which case you may want to buy there after all.
Advantage to buying at Sanibel -- You can reserve via HGVC at your home resort up to 12 months in advance. For the rest of us who don't own there, it's 9 months. And presumably, in the years you want to use Sanibel, you wouldn't even convert to HGVC points -- just use your Sanibel week.
Disadvantage -- Converting affiliates to HGVC points is not as convenient as using points from an HGVC-owned property. Check out my post about this
here. It's been discussed elsewhere on this board, too. You may have to browse through a lot of threads to find this subject.
Seth or Judi would be good resources to talk to about this. My take would be this -- if you plan to go to Sanibel fairly often, buy there. If you plan to mostly convert to HGVC points and travel elsewhere, buy an HGVC-owned property.