Angelique posted an easy solution for the problem already. Simply don't give anything away that you didn't create yourself (other than the one and only copy you purchased). Arguments like "a friend gave it to me, how should I know he stole it" or "the creator did steal somewhere too" are meaningless - it's up to you to make sure that you don't pass on pirate copies, period.
It is not that simple. In Second Life there is a tradition of creators making freebies with the intention that they be redistibuted. These freebies are usually set to full perms or to copy/no mod/transfer. There are thousands of such freebies available in the game. While textures are a special case, taking the "easy solution" outlined above means ignoring the wishes of the creators in the overwhelming majority of (non-texture) cases.
There is a sensible and responsible middle ground in Second Life between the extremes of RIAA-style distortions of copyright law and "if it I have it I can do whatever I want!".