It depends on the license of the model that you want to buy.
In sketchfab there are 2 types of licenses; a) Editorial, and; b) Standard. Editorial doesn't let you do much, not even commercial use, so I don't understand why anybody would like to buy it in the first place. The second one, Standard licence, let you use the model in (almost) any way, commercial included.
It's my suggestion that you should only use sketchfab as reference. If you want free models than can be used on CSP, or modify in 3d programs, you can search some models on Free 3D. There are very few free models, that's true, but if you want to modify some, this models can be a good starting point, so you have something to start with. It can be very time saving.
Also, if you learn to use SketchUp, they have a big warehouse and you can use most of their library for commercial use, and of course you are allowed to play with their assets and modify it to your needs.