Ex Capillus are "not reportable" in terms of eBay's sales. They are, indeed, First Class relics, but eBay allows the sale of hair so reporting that type is of no avail. Fingernails, I would imagine, are reportable.
Writing to Bishops is critical, if only to make certain that there is a proviso in contracts of sale of Church goods that would prevent things like the altar stones being sold. These guys got them somewhere, and that somewhere is a church that has been closed down.
I don't think it would do much good to write to them about relics.
They are available in abundance. I don't know about the present day, but just a few years ago in Rome, you'd find piles of them, tossed onto a blanket in the outdoor market places. I buy lots of Catholic artifacts at estate sales, and there are often first class relics tucked into drawers, shoeboxes, hanging from bedposts. Fortunately I was able to donate them all to a parish that has a wonderful reliquary cabinet in the sacristy. (I also found a pyx full of hosts in the estate of a deceased former nun.
but that's another story)
The point about eBay is that it is the largest international marketplace and just about anything is for sale. The guild came together in 2005 after someone tried to sell what he alleged to be the Eucharist consecrated by Pope JP II. It gave legitimate Catholic sellers a black eye. Selling the flesh and bones of those who have given their lives in a remarkable way for the Church is simply a disgrace and a scandal. It is one small way in which we can make a little bit of a difference.
Right now we're working on seals with embedded links to members sections on our site. Once that is complete -- probably after Easter - then we'll do some press releases to local dioceses with members in those localities as a focus.
I suppose e-Bay allows the selling of hair in order to allow the sale of mourning jewelry and pictures made of hair taken from a dead family member before burial. I know these were popular for decades, but feel confident in assuming that those things were never made with any expectation that they would end up being sold to someone with no relationship to the family. Therefore, I think they shouldn't be sold but should be donated to museums.
Anyway, I was actually thinking of two things, one of them being asking bishops to do as you suggested and make sure altar stones don't end up in the wrong hands. It would seem logical that a bishop should protect altar stones from closed churches within the cathedral of his see. If nothing else, they could be stored in the sacristy until a new church home can be found for them. (I'm fortunate to be in a diocese where new churches are being built all the time; surely it's not the only one?)
The other thing a bishop could do, and the USCCB could do as a group, is inform the public, particularly Catholics, that first class relics of saints, which are human remains, are not being accorded the same respect as the remains of Native Americans by e-Bay. Probably very few Catholics are aware of this.
I think e-Bay's policy of respect for Native American remains is the right thing to do, but it should apply to all human remains. Saints are not merely of religious importance, they are also someone's family members. Celibate saints who practiced chastity have no direct descendants, of course, but members of their family are very likely to have had descendants. How many saints had no siblings and no cousins with descendants?
I'm very glad that you are setting up a guild of Catholic merchants who not only know what should and what should not be sold but also know to investigate potential buyers to see if they have been buying items clearly intended for use in satanic rituals -- some scary stuff in that area. I imagine your members also won't be selling "third class relics" that were made in China and have no provenance of being touched to first or second class relics. ;-)