I did a factory reset to sell my phone, and my photos are still there according to the new owner… I need help in this strange situation! I did a factory reset and cleared everything in my HTC One X, erased all files and photos, removed all accounts, and sold the phone.

Now all my photos (2150!) are all back there, as the person who bought it says!
How is this possible? and what can I do to completely clear it up?
I made sure after the reset that nothing was in my phone!
If you thoroughly checked that nothing was left, that's a strange thing indeed (and I cannot imagine it returned "by itself"). But as a factory-reset usually only deletes/wipes /cache and /data, the SDCard (internal or external) are not touched in this process, so you might have missed that. Some devices offer to include the SDCard with the wipe/factory-reset, though.

For a real safe clean, however, even a wipe might not be enough, as files still could be restored using appropriate software. So if you're selling your device, you'd better make sure your data is completely nuked: not only "marked deleted", but really removed and overwritten. To achieve this, you can use helpers like Nuke My Phone.