It is not unusual now for eCommerce sites to offer a guest checkout for users who don’t register an account.
Card fraud is something to watch out for whether the user has an account or not.
Does registering an account make things significantly more secure? A fraudster can enter false details to an account. Though some sites that deal with more sensitive issues can have more robust account verification.
I don’t know the specific ins & outs of how Stripe works, but imagine, like other payment gateways, it will have algorithms to detect fraud, but like any system, won’t be infallible.
In the end, you need to the people processing orders to always be vigilant, checking the security status of transactions and looking out for anything suspicious. For physical goods to be shipped, checking for anomalies in the addresses can be a give-away. Though it is not unusual for legitimate buyers to use a different shipping address to the billing address. But you sometimes see things like a postcode that does not match the written address and suchlike.