Hi, everyone. I'm almost finished with MM1, so I thought I would do the remaining sidequests. However, I've encountered a huge glitch. Whenever I trade with the Hermit in area C3 for the Pirate Maps, it turns all the other items in my characters' inventories into useless items. If anyone knows something about this issue, any help would be very much appreciated, Thanks in advance!

It's a known glitch, also mentioned in some walkthroughs. I think he only replaces the items of the character in slot 1 so don't leave anything valuable there.

Although it feels dumb/broken, it's not a glitch but intended game design. The cluebook mentions it for example.

My solution was to create new characters. I gave all my useful backpack items to these new characters and left them in Sorpigal. With empty backpacks, I traded nothing for the maps and useless items, dropped all the useless items, and traded my backpack items from the new characters.