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.
Through corruption of the Dhamma comes corruption of the discipline, and from corruption of the discipline comes corruption of the Dhamma. This is the first future danger as yet unarisen that will arise in the future. You should recognize it and make an effort to prevent it. (AN 5.79)
Neither traceable in the Discourses nor verifiable by the Discipline, one must conclude thus: 'Certainly, this is not the Blessed One's utterance; this has been misunderstood by that bhikkhu — or by that community, or by those elders, or by that elder.' In that way, bhikkhus, you should reject it. (DN 16)