The hang up with that is it would double the shipping fees for each party. They would each need to pay for shipping their individual knives to the third party and then they would also have to pay for the third party to ship the knives to their respective final destinations. Now add on top of that the "service fee" to compensate the third party for his/her time both in shipping and record keeping, and gas money for driving to the PO for both items' pick up and shipping.
It could be done, but if both knives were able to be shipped in the smallest "if it fits, it ships priority mail boxes", that would be a minimum of $13.60 ($6.80@). If a minimum service fee of $10@ was implemented, that would leave $6.40 TOTAL for driving to the PO to pick up the incoming packages (delivery to a home mailing address would be risky), deliver the outgoing packages and track all 4 shipments. And even if there were multiple transactions passing in and out daily, it would still work out to way below minimum wage.
A good concept but unworkable when faced with the reality of trying to pry enough funds out of all parties to make it viable.