I have both bought and sold on ebay. My birds were selectively pen bred, fed breeders pellets (can you even get them still?) and carefully wrapped or hand delivered if local enough. I never used poly boxes as in my experience, they suffocate the eggs and the hatch is never as good.
Most of my purchases have been dissapointing, with poor shell quality (thin or patchy), hatch rates and dubious looking birds at the end. Posting always effects fertility (esp. in those blooming poly boxes). Some have been great, especially when collected. My most recent purchase is typical: 2x6 La Bresse eggs from two sellers of good repute but at the end of the season. The eggs were small and mediocre and I ended up with just 5 hatching. There was a spate a while ago of people buying eggs at auction and then selling them on, so they were old and very dubious.
Oh! I also had my listings for Old English Pheasant Fowl pulled twice!! Apparently for contraveneing Ebay rules somehow. I suspect it was actually other sellers of OEPF reporting them so they had less competition as there was nothing wrong with the listing at all.
I haven't bothered using it for selling recently, preferring Preloved.