I would only advise the selling of any sort of materia under very few circumstances. For one thing, you stand to gain very little profit from the selling of materia, unless you've gained a lot of AP for it, and if you've gained the materia a few levels it becomes much more valuable. It you can sell a materia for a considerable sum of money, it means it's close to being mastered. If that's the case, it makes more sense to get it to Master level, when a new materia will be born.

There are exceptions to this rule. One of them being the selling of All materia, which is the most common way of making large amounts of gil, because of the relatively little amount of AP required to master it, and because it sells for 1.4 million Gil at that level.