Let me get this right the key is 50 characters? And your doing this why? And why would anyone pay 0.001 for any part unless it's part 10. I think you havent thought this out properly.
The key is 52 chars, two of which are revealed. You need all parts to access the key, you can't pay for specific parts, they will be revealed when enough money is collected. If someone pays 0.01 outright, all parts will be unlocked, not just Part 10. They need to keep paying until the pool reaches 0.01. The parts will be revealed in order not randomly and you need all of them to put the private key back together.
Each part is a hash of 5 chars from the private keys, the final key will look like below:
K[Pt.1][Pt.2][Pt.3][Pt.4][Pt.5][Pt.6][Pt.7][Pt.8][Pt.9][Pt.10]g
K[XxXxX][XxXxX][XxXxX][XxXxX][XxXxX][XxXxX][XxXxX][XxXxX][XxXxX][XxXxX]gOnce a hashed part is revealed, all they have to do is crack the MD5 and get the 5 chars in plain-text which is a pretty easy job. if everyone waits until they get to part 10, the pool will never be filled.