There is also the question of whether you made those poser
skins yourself.
I have many poser and daz compatible skins, and it is possible
to rework those skins to work on an SL avatars.
But then the question is, do you have permission to do that?
You can check the documentation that comes with those products.
If you are not intending on selling those skins then the issue may be moot, it will once again depend on the perms the creator allows.
One thing that is true is that if you use a product like this
in your own rendered work then it is supposed to be ok to use it as your own.
Except there are questions which come into play when you extract the elements of the render for one specific purpose
such as the reproduction of the skin in order to sell the skin.
When I was learning about these things I emailed 3d content
creators I had questions about, (outside of SL).
And what I found out is that there are quite a few things that
are technically legal which are not necessarliy ethical within
the artist community.
From what I have read here and elsewhere the best thing to
do is learn how to make them yourself which for sure takes
more effort.
But is much more rewarding and gives you a marketable skill as well.
