Don't give anything away for free without fully vetting the people you're giving it away to. You're leaving yourself open to attacks by trolls and dubious members. This is an illegal drugs marketplace, not Sainsburys. Not everyone on here is a good guy, and some people are not who they appear to be.
Most established vendors have had unfair or patently fake reviews. It's very possible that some dodgy types will take your free samples just to shoot you down. It's happened before, and will happen again.
My advice would be to offer some samples at a low price, but not free, and offer them to established, reputable and most importantly, impartial members who aren't obviously attached to specific vendors.
Up to you though. Just my 2p.