I believe you can flag scalpers on eBay and other sites so they get taken down. Unfortunately, there is always going to be another site to sell on and someone will pay the money.
The problem isn't so much the online selling. The problem is that Wal-Mart, Best Buy, etc are not doing anything to prevent shitbag scalpers from using bots to buy from their sites.
1 - Person A buys a PS5, sells to Person B at a higher price (limited PS5 supply)
2 - Person A buys a PS5 and keeps it, Person B buys a PS5 separately (ample PS5 supply)
3 - Person A buys a PS5 at MSRP and keeps it, Person B buys a PS5 at a higher price but from retail (adequate supply)
The current situation is 1. The ideal situation for consumers is 2. The ideal situation for retailers is 3. I think 1 and 2 are obvious, but let's think about 3 a bit. You know those annoying $800 bundles that GameStop and other retailers always push? Those are aimed at Person B. If the PS5 is a huge hit and it becomes hard to get UNLESS you buy an expensive bundle from GameStop then Person B is more likely to buy that bundle from GameStop (and get some games and accessories for that extra money) than from some random Person A on eBay (and only get the console).
All of this is ignoring the position of Sony. Ideally for Sony, the PS5 would be priced just right so that every consumer is giving the most money possible to Sony for the PS5, but the realities of retail distribution and selling a mass-market device limits that.