Buying the model from some source is totally legit, it’s also something Iris simulations does (a reason they skipped the C-17 for now as the same model was bought for the freeware version).
However what’s not okay to me is to just use the flight dynamics, systems and sounds from default aircraft straight away, do no optimization to the model and ask to get 30+ bucks for this which basically is half the price of MSFS standard edition again.
Developers like Iris at least take care of the systems, optimizations on the modelas well as custom textures and sounds so the price they ask for fits the effort.