This is different then selling a buddy pass to a specific buyer. This person is selling his/her "friendship" for one year. Being this person's "friend" carries the privilege of being on their travel benefits, which is now possible because of the ramifications of the domerstic partners obtaining travel rights of an employee. Once the travel benefits were opened up to people outside the traditional family (Wife, Children, Mother and Father (or in-laws)) this type of arrangement became legally possible.
As someone who comes from what you'd obviously cosider a "non-traditional" family, I can't help but sense a bit of bigotry, or at the very least ignorance in your post. You must be unfamiliar with the difference between a Domestic Partner and a Registered Guest.
A Domestic Partner is considered the same as a spouse and extends beyond travel to insurance, bereavement, and more. Documentation must be produced, much more than is required from a married couple. US Airways is unique in that they recognize both same-sex and opposite-sex partners (most companies take the stance that opposite-sex partners can marry if they wish to receive benefits). It is not an easy or casual thing. A friend, fling, or lay of the week could not pass for a Domestic Partner, nor could an internet bidder.
US Airways is also unique in that it offers a Registered Guest program to single employees. The RG program is for travel only, and the employee can add anyone they wish. Say what you will about US, but they have hands-down the most generous travel program of the majors. It is also the most fair in that it does not favor married employees (state-recongnized or not) over single employees. They are not legally required to offer anything to anyone as far as travel priveges are concerned.
I seriously doubt this person is offering to produce proof of shared financial reponsibility, shared residency, and notarize legal documents declaring partnership, and then pay for the health insurance of a stranger on Ebay. They are clearly offering thier Registered Guest Pass. So, enlighten me, what on earth does this have to do with the "ramifications of domestic partners obtaining travel" or "people outside the traditional family?"
I think this person is a fool, but it's not like people don't get something in return from thier registered guest. Reciprocal travel benefits from an employee of another airline, free food or drink at a friend's bar, money, whatever. Most people just don't literally broadcast it over the internet or make deals trusting thier travel privleges with a complete stranger.