generally if you are not allowed to sell models it will say so in the liscence agreement, you would usually find this sort of thing with generators like Poser/Terragen/Bryce, or student/Learning editions of programs,
with stuff like Maya Personal learning edition you can save and not export, so its purely obvious there is no commercial use, but student versions of proggies like 3DS Max, C4D, Maya etc which are usually a lot cheaper than the full priced package, the only restrictions with them is simply commerial use.
Free apps and fully bought 3D modelling/animating apps that have full liscences will allow for you to use models commercially, I like the idea of selling 3D models, however as soon as I started modelling I thought of making something similar to dark matter, slightly over 1 year later, haven't even modelled something to sell, hopefully your addiction for improving your 3D modelling won't get in the way of greed