Sounds like a good idea, just so long as everyone understands they participate in such trades at their own risk, and that Dhamma Wheel is not responsible for any unsatisfactory consequences that might arise because a trade doesn't pan out as intended. I also think that it's reasonable if people just want to give books away that they can ask the receipient to pay for the postage if they wish to do so.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"Whether I were to preach in brief, Sāriputta, or whether I were to preach in detail, Sāriputta, or whether I were to preach both in brief or in detail, Sāriputta, rare are those who understand." (A I 333, Sāriputtasutta)