If a resort has ROFR, the seller, or their agent, must offer the timeshare to Starwood before the sale can be completed. If Starwood chooses to exercise ROFR, they pay the seller the same price, and the buyer simply walks away.
There is no way to know at what price Starwood will exercise ROFR, except anecdotal evidence, they don't reveal that info.
As a buyer - you have nothing to lose by making an offer on a resale - if Starwood exercises ROFR - you lose nothing but a little time. However, Starwood cherry picks the best deals - the definitely don't exercise ROFR on all sales.