I observe that the use of 3d max is fundamental in the industry and I wonder if this determines the success in the sale of 3D models...

Agree with PhantomG, Max is not necessary. Any purchaser who wants an object for use in MAX will purchase a model in *.fbx file format. You are offering your models in *.fbx format, so you are accomodating the pool of MAX purchasers. There is no need for you to acquire MAX as well.

3d max isn't fundamental in 3d industry, it doesn't determines sale success, what determines number of sales is the quality of the model itself, it doesn't matter in wich software it's done. I never comment/rate other artists work because it's ungrateful, something that is good for me doesn't necessarily mean that it's good to someone else and vice versa, one thing i can tell you is that your prices are low. Don't sell hard work cheap, if you do, you and your work won't be respected.