I like that idea..
Except for the fact that eventually there will be so much materia that the markets are flooded and people would gladly put up 25 materia's to have that 5th materia 100%

What most people havent realised yet is that boosting the stats on your tools using materia helps the percentage quite a lot.

Instead of 7% for the third you can get it up to 10% with a couple of control materia.