You have two questions which are so broad there's almost no place to start. Therefore the place to start, as others have suggested, is TUG.
Buying timeshares generally
You have a steep learning curve ahead of you, because every timeshare is different -- both in terms of resort(s) and the operational details that will make it work, or not work, for your family. I'd suggest that you narrow the industry down to 2-3 of the top companies and then thoroughly research all of them until you really understand what they will provide you, how they work, and what they will cost you, both initially and each year.
TUG is the best place to begin gathering that information. Here on the DIS, there are a number of very knowledgeable and helpful members who can also help. When I was doing my research, I relied heavily on DIS members whose posts here I'd read for years and had great confidence in.
Buying on eBay
The default position for almost every experienced timeshare owner is buy RESALE, never direct from the developer. (And, like every rule, I'm sure there are excepetions -- I just haven't found one yet.)
eBay is one of the places to do that, but not the only place. On TUG, you will find several realtors who sell timeshares resale and also a number of direct sales offered by owners.
I purchased my Wyndham contract resale on eBay. Buying on eBay requires going to school on how eBay timeshare resales work. Generally, the price is set in the last ten seconds of an auction by automatic bids and everything that happens prior to that is just noise (unless the bidding indicates the contract will sell higher than your acceptable range). It's very important to set an upper limit and not get carried away by the excitement of an auction.
There are people out there who shop on eBay for you and they have some expertise in that process. They can be a great help in imposing discipline on the auction process. I don't know what they charge; I did my purchase alone.
No matter how or where you buy, you should NEVER, EVER pay any upfront fee to anyone. A deposit with an offer, yes, but you should not pay any fees in advance of an actual purchase. If someone asks you for an advanced fee, it will be a scam 99% of the time.
Good luck and happy hunting!