I guess I've been lucky because I've never had a truly bad experience buying online.
I once bought a Rubik's Magic Snake which turned out to be a cheap copy, but in all fairness, the seller listed what country it was from and showed the packaging. I just assumed that it was an official product.
I also bought a couple games, CD-only, from one seller with a lot of complaints from buyers saying that they received broken discs and in each case, the seller told them that they should have paid for insurance. I paid for the insurance and the discs arrived intact, albeit just stuffed into a padded envelope with nothing extra to protect them from breakage.
A few years ago, I bought a CPU from a seller on Amazon and they sent the wrong chip. When I contacted them, they apologized, said that they didn't have the one I needed and told me to keep it. It was only like $10, but I thought that was nice.
Many years ago, I used to collect video game cartridges and I bought an Intellivision Pinball cartridge from one guy. It came, I plugged it in, turned on the system and was greeted with Atarisoft Pac-Man! I contacted the seller and he said he hadn't tested the carts because he wanted to send them out mint. I asked him to please test one to verify that it was Pinball and send it to me. He did so, and sent it out immediately without waiting for me to send the other one back (which I did).