The TUG (Timeshare User Group) mentioned in earlier posts is the best place to start. I have bought Marriott resale and it is no problem.Don't get in a hurry. Some options are buying resale from Marriott(poor choice IMHO),individuals who sell through sites like Ebay and Myresortnetwork.com,or from local agents who specialize in certain markets in timeshare. There are several of these in Hilton Head for example.
Buying resale is liking buying airplane tickets. Thesre are a lot of people sitting next to each other who have paid different prices. It is the same in timeshare.