Upload models with modifiers.

I think everything is possible as long as your model is properly described. Because if your model uses a lot of program-specific features, it is much less interchangeable with other applications, and potentiels buyers must be warned before their purchase. You can also put the two versions in your package (with modifiers for native users and frozen for interchange especially if You can add .obj and .fbx to reach a wider audience), which would enhance the value of your product.

I think it's acceptable to leave some modifiers uncollapsed, as long as those modifiers are native and not from plugins. Just don't leave your entire modifier history from starting primitive to complete model uncollapsed. Leave only those modifiers that could be helpful in adapting your model to buyers needs. And don't forget to reset transformations of your models before finalising.