I recently bought a Curta Type I on ebay. This is my first time buying a Curta. Appearance wise, it is in great condition. As for functionality, all operations work well except for one - the clearing. The description of this Curta on ebay said that it has a clearing ring to be replaced, and I thought that was it. But when I tried to clear the thing, the clearing lever seems to be stuck in its position and refuses to turn (I did lift up the carraige, so that isn't the problem). Can any Curta enthusiasts help me diagnose this problem and tell me what may potentially be wrong? I'm willing to take it apart, but is it out of the reach of an ordinary person (I'm no mechanical expert)? Any feedback is much appreciated!

Is the clearing plate completely frozen in place or can it still wiggle a little bit? If it still can wiggle then I suspect that the tiny clearing detent plunger is bent and cannot move down into its bushing. That's why you can't turn the clearing plate.
To get to the plunger, you have to take the crank off, remove the bigger/lower circlip, remove the carriage, unscrew the center nut on the inside of the carriage that holds the clearing disc, remove the clearing plate, and then you can see the plunger. Check if it is moving. If not, try to carefully remove it and if it's bent, try to straighten it.

Guess why I know this....

This helped me a lot when I fixed my bent clearing plunger:
http://www.vcalc.net/disassy/

Thanks for the feedback!

Mine can still wiggle a little, so I guess it's similar to what you have encountered. I'll have to look for the tools to disassemble it.

Once I have the thing taken apart and hopefully fixed, I will post an update!

Finally got around to fixing it! And just as Andre described, it was the plunger that was causing the trouble. Here's an image of it:

After bending it straight and checking if it can go down into its bushing, I assembled the curta back together and it worked like a charm! Here's a photo of the plunger in its bushing for reference:

Thanks again for the help!