I had similar questions in my mind last week when I saw classifieds about sellers wanting to trade or sell their used soap and cream collections. I was trying to look at it from a vendor's point of view: would a vendor have a try-and-buy policy, where they let you try a soap and accept its return if it was "only used once" and you didn't like it, and restock it for another buyer to try-and-buy later? I came to the conclusion: NO. It's all about contamination, whether it's intentional or unintentional. You never know what you'll end up with on used soap, so avoid that secondhand stuff.

On a similar topic:
How about this, if you were taking a shower at a friend-of-a-friend's home, would you bath with their used bar of soap if you didn't have any soap of your own?