Two I can think of, luckily neither was that bad.

Firstly I got impatient and bought a new DSLR camera kit off eBay, thinking I would save money and get a good deal. It came with two decent lenses, supposedly, and a bunch of other accessories. Very highly rated seller.

After I made the purchase, I get a message to expect a phone call from such-and-such number. Strange, I thought. They call and immediately I can tell it's a bait-and-switch. They tell me what they're going to send, but it's not what was in the listing. Only one lens, instead of two, and some other shenanigans like substituting inferior brands and cheap shit. I called them out and said either you deliver what was promised in the listing, or I'm opening a dispute, and it won't be a good look that you tried to change the deal over the phone.

Anyway I got what was listed, but overall it was a disappointment. Grey market items from overseas, not official US licensed gear, so I had no warranty. But I ended up paying as much or more than if I had walked into a local shop. It wasn't counterfeit, but just left a bad taste in my mouth. The seller disappeared from eBay not long after that...

Second time: I received one of those emails with a password in the subject. It looked familiar, and was in fact an (old) password I had used. Someone took a hacked DB and just fired off countless emails with the passwords to the matching email addresses. But the tone of the email was what spooked me. It said, I have had full access to all your emails, I have figured out how to reset accounts and hacked into your webcam and have some very interesting photos. Either you pay this amount to this bitcoin address or I send the photos to all your contacts and your life will be ruined.

In the moment, I panicked like oh shit this is legit. Even though I couldn't imagine what photos they referred to, it was still scary being blackmailed. I thought about it, discussed with some people, and they helped calm me down. After a few days, I realized it must be a scam. It was so generic. Surely if it was real, they would mentioned specifics... my name, or what I looked like, or some other unmistakable details.

Over the years, I received a number of other variations with the same jist, and different passwords (my email address was in several major leaks in mid-2000s). I'm glad I didn't fall for that shit, regardless of how serious it seemed in the moment.