I know that most sellers on aquabid have a DOA policy. For example, when I purchased my betta, the seller had on the auction page his DOA policy. It stated that if the fish was dead upon arrival, I should take a clear picture of the dead fish in the unopened bag and email him the image. Then the seller would give me either a refund or credit for a new fish. Some sellers only have live arrival guarantee with fish shipped overnight. If the fish dies within a few hours of you adding the fish to the tank, most sellers will not assume responsibility. However, I have purchased eleven fish online, and not a single one of them has shown a sign of illness. You can message individual sellers for more precise information, and many of them are happy to explain it.
Also, there are always people selling killifish, so I think you will have good luck on aquabid.