Spiritbonding and selling materia would be the best if you have a lot of time to exp at 50. Savage Might IV should sell for tons and you can get them fairly regularly if you are converting the right stuff. There are a few other materias that are high sellers as well. At the end of the day, this method is hit or miss, though.

If you're looking for regular income, I would suggest DoL as others have mentioned.