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.
Metta, Retro.
If you have asked me of the origination of unease, then I shall explain it to you in accordance with my understanding: Whatever various forms of unease there are in the world, They originate founded in encumbering accumulation. (Pārāyanavagga)
'We should not congratulate someone on the success of their misdeeds, but on the contrary should endeavour to advise him or her to lead a more skilful and wholesome life. If such advice is ignored then we can only give up and let go' - Phra Panyapatipo