You can buy a resale nearly anywhere, and the process is pretty much the same for any resale. Is there something in particular you are wondering about, like buying on ebay?
FIRST - Click on the ADVICE link in the red bar, and read some of the articles about how to buy on the resale market.
1. Check out the reputation of the ebay seller on TUG - BEFORE bidding.
2. Use the link on ebay to ask the seller all your questions before you bid.
3. If you win the auction, do not pay upfront - ask for the estoppel letter first to verify all the details.
4. If the details do not match - you have the legal right to walk away from the deal with no consequences.
5. Use a reputable closing company to handle all the details - that means checking out the sellers closing company before you bid.
You don't have to buy on ebay - you can also buy in the TUG Marketplace, Redweek, Myresortnetwork, etc.
You can also use a reputable broker - see the ratings forum which is a sub-forum of Buying Selling Renting. That will cost you more, but they will walk you right through the whole process.