Register for alerts
The best way to prevent a bogus transaction is to set up email or SMS alerts, as they will at least give you a warning as to when a transaction is made or tried. And if the said transaction is not by you then you can take action immediately.
Don't save your card information on websites
It's not foolproof but it would certainly clog some loopholes. It's better to limit the sites where you save your card details and know all the sites you have them saved on. Best to save them on trustable sites.
Be careful
The Internet is full of baits so be prudent while clicking on any too-good-to-be-true deals. Especially the ones that ask for your card details. Be paranoid of fishy email links and consider them as red flags.
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