I have thought of doing this too. I have met some people who actually do it but they are pretty well known in the scene, I guess finding a market to buy your cumrag or your stinky socks is a privilege of more famous people lol
However, my general feeling is that you should be able to find buyers no matter what, as soon as you are quite persistent with it. Many fetishists exist. And I can tell you by experience, I have thought of buying some used undies too (the only reason I haven't done it is because I don't live alone and I'm pretty scared of others finding what's in my mail lol)
As for safety, I think it's possible to send a letter/package without a sender address (or at least the sender address will be kept private by the post services). At least that's how it works here. The best thing is to go and ask!