Likewise I think the idea is first class. From what I have seen, Mayhemer's are an honest bunch, and the likelihood of something 'going wrong' is remote. However, there could easily be one hard and fast rule: If either party is not happy, the exchange is cancelled without question and the items sent back.
If I might have 1:96 3 rail stanchions to swap for 1:96 general deck fittings for WW2 GB destroyers, does it mean I have to p.graph each one of them? In the same light, does the vendor have to p.graph each an every deck fitting? Both parties would know exactly what the items would look like once the mfr. was known, so there is no need really for p.graphs in the first place as both and everybody knows what the things look like. Quality is the only real issue, but this is between the two parties and comes down to new and used. If a Mayhemer said an item was new and Deans or Haynes mfd. I would know exactly what to expect, if used, well "you does your swap you takes your chance".
At the end of the day the Rule mentioned in my first para. would carry everything. No exceptions, no further griping, just all returned and that's that. I do think it's a good idea, it will be interesting to see how it develops.
Regards, Bernard