Many of us like eBay. Just go with a seller with a lot of positive feedback. If you find something appealing, post it here in the appropriate forum. Owners will usually give you an assessment of whether it's worth chasing.
Look through the TUG marketplace (up in the red stripe above) and in the subforum Bargain Deals in Buying, Selling, Renting forum. TUGgers often post their own unwanted weeks and/or weeks owned by parents/friends.
I don't care much for Craigs List. It's just locals trying to sell their own. No way to tell who's reliable or not. Nothing verifiable.
I have bought from both TUG and eBay and had a satisfactory experience. The eBay one I outbid a $1.00 offer and landed it for $1.25, and the TUG marketplace one was just exactly the fixed week I wanted in the resort I wanted. It was not the bargain that the other one was, but is eminently usable, and if I want to, outstandingly trade-able and salable.
I again ask that you take some time. Learn about the various systems. You wouldn't buy a car, investment or anything else that can cost you thou$and$ a year without understanding it and making an informed decision. Don't buy a timeshare on impulse. The bargains aren't going anywhere. You can rent weeks from owners for about the cost of MF,(or less) with no commitment, so do that a few times. Find out about the infamous Wyndham 'parking pass in exchange for a sales meeting' fiasco. Learn about salesweasles lies.
Have fun, learn much. I had lurked her 5 years before I was ready to buy, and STILL made mistakes! So don't rush into it.
Best wishes.
Jim