Some you do some you don’t.
Renderosity does sell items but there is a ton of free stuff also available in a lot of different formats.
You can search under free stuff > whats new. Under whats new just type in the search box what you are looking for.
What you are looking for as to licensing is something like this. (you might need to be registered to see it)
Note the license states “This is licensed for commercial or non-comercial use” so it’s CC-Zero.
By the way if someones after free mocap animations.
Not fit to finish so a fair amount of clean up will be needed but it’s a good start.
Another place to look is here
The majority of their stuff is marked as “Personal use only License”
There are a few different formats available at Renderosity but by far anything available with in Poser or Daz Studio format is where you want to look as it is the community that has been giving stuff away for free because that’s what they do. The odds you would be sued is zero.
As to OBJ the base mesh as part of the package, assuming just props, is usually in OBJ format as part of the package but I would suggest that you download DazStudio, free once again, to catalog and stage your goodies properly. From there you can send it out to any place you like either FBX or OBJ.