You just opened a can of worms
The problem is that you’re comparing totally different scenarios and different businesses.
Amazon is the biggest webstore in the world so you can’t compare them with anyone, not even with their big “competitors” because Amazon is pretty much eating them alive for breakfast. Oh and they also operate with billions of dollars, so $1, $1000, or even $100,000 isn’t going to harm them in any way to start forcing their customers with some intrusive ways of verifying themselves.
Also, corporations such as Amazon get to have direct relations with the manufacturers (mfs. in our case being the companies that issues these gift cards such as Apple, Google, Sony, Microsoft, Nintendo…etc) of almost every product that is being sold on Amazon’s store, and if not, then they are dealing directly with the official distributors of those manufacturers (talking about products in general that are being sold by Amazon themselves, not a product that is being sold from a marketplace seller). Amazon doesn’t allow marketplace sellers to sell any digital goods delivered by email, so that’s only reserved for them. Amazon also uses GiftCardMall’s services (that is owned by Blackhawk Network, a major gift card corporation) for most of the third-party gift cards that Amazon sells. So if there were any issues involved with any orders from scamming customers, Amazon can simply reach out to their service provider and ask, for example, to cancel/revoke such gift cards as they were purchased by an unauthorized person. Then the service provider can contact the gift card manufacturer (let’s say Sony) and have those gift cards revoked from your PlayStation’s account (or they may even have the power to do such things without having to contact Sony about every gift card), which means it could also result in having your PlayStation account getting suspended.
Steam doesn’t sell third-party gift cards and they don’t sell cash-like products (yes, in most cases gift cards are considered as cash). Steam only sells their own Wallet gift cards, so they are the manufacturers of their own product which grants them the ability to control every aspect of it. Steam also has the power to revoke anything in your account even after years of you buying something with a CC that you stole years ago, after that card owner goes and files a chargeback and wins the case with his bank. Steam also has the right to ban you from using their services if you commit such crimes.
Gameflip, on the other hand, doesn’t have the power to revoke your PSN or Xbox gift card that you purchased, nor they have direct relations with any of those companies. So they need to introduce such requirements to absolutely make sure that you are authorized to use that payment method. And even then, it’s still not a sold way of proving that you are. Gameflip also doesn’t have millions to spare like Jeff Bezos and Gabe Newell. It’s just like you going ahead and creating your own marketplace and allowing people to sell on it. Are you going to let scammers have it easy to cheat people and get away with it? No, you’ll have to take responsibility for some things, and in order to save yourself from going bankrupt, you try to limit the risks the best you can.
Almost every other online store/marketplace that sells such digital contents that I know, will have similar requirements to limit the risks of taking a big hit from scammers. I could count at least 10 names for you, but it’s not allowed to mention other stores around here.
So yeah, I hope now you can think about it from a business point of view and put yourself in their place to see why you have to ask for such forms of verifications
BTW, you only have to verify yourself once and then you can buy whatever you want on GF. If you’re not comfortable with doing this, that’s ok because no one is forcing you to buy from them. But if you want to save lots of cash, I suggest you go ahead and verify your identity so they can accept your orders. I already paid over $100,000 to GF and I couldn’t be happier.