I think it is pretty rare that something like this would happen, bids and a Buy Now. But having said that we allow people to list domains whatever way they want. Ultimately if the domain sells you are responsible to give that name to the winner. If you list it two ways and sell it two times and obviously cannot deliver it to two parties we ban you from the auctions for failing to move it to whoever didn't get it. So you can do it but you are playing with fire.
I don't see this as being much different than listing it with a BIN on multiple platforms. Say GoDaddy, Sedo, Efty landing page. You sell the same domain in two of those places at the same time whichever one you dont complete the sale with is going to do something to you like fine or ban you. That is the current state of the industry and we have chosen to not ask for exclusivity, even among our various listing types, or on other platforms. But if you choose to do that there is some risk.