I don't think it's possible to retain StarOptions and sell the associated interval. I don't believe they can be separated like that.
I think you are probably worried because the Ebay ad said no StarOptions. Is that right?
I think it is highly likely that the seller (who presumably only owned this for a short time) does not understand what they own. I think it is quite likely that the StarOptions transfer with the sale. I wouldn't worry too much if I were you.
The only situation that I can think of where you might have a problem (and I think this is highly unlikely) is if the previous owner renounced their membership in SVN. This is not easy to do, but there has been at least one person on this board who has done so (she owned Vistana Resort and wanted to drop SVN). I don't think many people do that, since membership in SVN is considered to be quite valuable by most people.
My understanding is that the word "mandatory" means that SVN membership is optional for resale buyers, but there is an assumption that the new owner will join. You would have to specifically ask them not to include it you didn't want it. If that is the proper definition, then I would argue that, even if the previous owner did not choose to join SVN, you should be allowed to.
Bottom line is, I think you are going to be fine. You picked up a great timeshare at quite a discount to developer pricing. Pat yourself on the back and get ready to enjoy it. And even if there is some screwup with the StarOptions, don't worry-- I think you will find that you will be a very popular person on the Starwood direct exchange board (there are very few WSJ weeks there).