I've had people bid on a couple of pieces of mine and I try to keep meticulous track of what I use in each scene (It's a PITA, but it saves me from trouble later.)
 
99.9% of the time, if I contact a creator and ask nicely not only do they give permission, 9 times out of 10 they don't want anything from me. (I put back funds for them regardless and it is always apreciated--hey, who doesn't like "found money"!)

So, to me, I don't worry about restrictions for use as most of the time I don't render anything for sale and if someone expresses an interest, I can always contact the content creator and ask.

Perhaps that is the key thing, ask.

I always give thanks to the freebie creators. I remember when I started with poser, I didn't buy hardly anything, freebies were my friends! Now I have accumulated a fairly large "purchased" load of goods, but there are still those favorite freebies that just "make" a render work.

Freebie content creators fill a much needed area, and they are my heros if for no other reason than I can't model my way out of a wet paper sack!  ;-)