Excaliber,
If you're that worried, I think you're not at all comfortable with the deal and the other person. When I did my first ball exchange, I was kind of worried; then I figured -
Hey, if I trust him, I trust him; If I don't, I shouldn't do the deal. I shipped first, then when he got my balls (if was a multi-ball swap), he was surprised and admitted he wasn't sure about me and had been hesitating.
I'm not saying you should trust everyone; in fact, based on the number of problems reported here lately, I wonder about trusting almost anyone, but there's only so many precautions one can take in life. $50-100 lost is a painful experience but it's not the end of the world. If you've taken all the measures you can, made all the checks that are REASONABLE, take the plunge.
Don't be a fool, but life without some degree of trust is not worth living.