I hope most of us here appreciate the ethics at play here. I think that if we were to examine the mission of the organization, it would likely be to provide as many as possible with serviceable bicycles. This mission is furthered by selling high value donations, and using the proceeds to acquire multiple lower-value donations. As long as the organization isn't abusing the trust of the donors by skimming donations for personal gain, I don't think there's a problem. In fact, Bicycles for the World does exactly this - selling high value donations at what the market will bear, and plowing the proceeds back into the organization.