Actually eBay themselves say the opposite – from their site,
“Many problems are misunderstandings that can be worked out when members talk to one another. The first thing we recommend is for the buyer and seller to communicate…
If you aren’t able to work things out after communicating with your buyer or seller, you can contact us.”
It seems they say if you can’t work it out, they should arbitrate, not a beis din. OTOH, maybe agreeing in advance to a specific arbitrator would be considered communication and working it out…
Firstly eBay accepts the right of a buyer to cancel – there’s just the issue of potential negative feedback and potential legal consequences of breach of contract which here would not apply due to the terms explicitly excluding anyone other than Jews and secondly the seller would almost certainly win in a Beis Din in demanding that the buyer cancel his bid.
I think it’s unlikely that eBay would be okay with discrimination as long as it’s spelled out in advance. It would also be unlikely for someone who’s not a frum Yid to accept a psak from a beis din.