Wax is the company that runs the platform. They have their own coin/token, similar to bitcoin on any other blockchain currency. Topps cards can be purchased for Wax coins and are digital goods on the platform. Ebay does not represent all sales of virtual GPK.
Blockchain currencies are more secure than your social security number, bank account, etc. As long as you store your wallet info securely, you are fine. In fact, some would say currencies like Bitcoin are too secure, as there as now way to retrieve your coins if you lose your wallet info. There were early miners that mined a few hundred bitcoin when it first started, then threw out their hard drives/computers that contained their wallets. One bitcoin is currently worth $9,200, so there are millions of dollars worth of them that are gone forever.
You can read more about how block chain currencies work here:
https://en.wikipedia.org/wiki/BlockchainPersonally, I would not consider these cards to be a good long-term investment at this time. Digital currencies and items tend to shoot up quickly at launch and then slowly come back down. Getting in at launch and then getting out at the peak is the way to do things.