It shouldn't be hard to convert models to max format, but materials usually have to be reworked from scratch and that can be quite big task. But even if you'd find someone, who would convert your models to max, how you're planning to provide support for those models? Lets say customer bought your model solely because of max format and he discovered some issues with the model or file, how will you fix those issues? Selling models in formats for which you can't provide support, is a straight road to refunds and negative revies. Ask yourself if you want that?