I have owned one for ten years, and it has worked very well.
Here is how I went about it.
Timeshares should never be viewed as an investment. Period !!! Chances are you will not get your money out of them. If you don't have the disposal cash to vacation and travel, you still wont after buying a timeshare. Take a very clear look at your life and financial position, and not while you are enjoying a beach or paradise somewhere.
If you didn't travel before, this will not change after the purchase. Evaluate your previous travel history. If you only take a vacation every third or fourth year, this will not change because of a timeshare purchase.
Never buy from the actual property, always buy on the after market. I paid $2800 for a property that was going for $12,000 at the time. This is very important. Contact the actual property before signing the deal. They might charge you some money to do the research, but make sure that the deed is clear for transfer and the tranfer is approved before closing the deal. There might be hidden costs for a transfer. Sometimes they are not allowed. After market purchases are risky, do the research.
Look for low yearly fees. Mine are less then $200 per year for an every other year week. I make up more than the $200 yearly fees in bonus time discounts. Mine allowes me to book bonus rooms for my family and friends. This ammounts to about 20 nights per year, at a fifty to sixty dollar discount per night.
I prefer to have a kitchen and full suite when I travel. Memberships through a timeshare make this possible.
You will not save money through the use of timeshares. You can book travel packages cheaper. Very often these packages cost less then the flights will cost to book separately.
From observations of friends. If you can't pay for the timeshare in cash, you will hate making the monthly payment, and it will ruin your vacation.
Everything listed above works for the points programs, but they are harder to find a deal.
I have not worked a timeshare deal through ebay, but many very reputable real estate companies have them listed. For Mexico properties the prices are about a quarter of what they are at the property.
Once again, be very careful with an after market purchase, but it can be done. I would say that in most cases a timeshare is a waste of money, and you can travel cheaper and easier other ways. But in the right circumstances they work great.