You could offer to replace eggs one time if they pay shipping, but I would not agree to the refund. You might want to update your wording on your sale page if you decide to keep selling eggs. You definitely want to state that the hatching success is not guaranteed and the only thing is guaranteed is that they will receive fresh fertile eggs, once they leave your hands and are at the mercy of the post office you can't guarantee a stress free shipping trip (sounds like you already have something like this). I suggest adding wording that the buyer may want to take pictures or a video when they unbox the eggs, so if there was an actual issue there's clear documentation, and have a stated policy for such issues. Same for a "poor hatch" complaint, request pictures of the opened eggs so you can tell if it's an actual issue with the eggs, the incubator, or if they're trying to scam you. You might take a look at other sellers on eBay to see what wording they use.
I've bought quail eggs on eBay and had poor, decent, and great hatches. I've bought chicken eggs on eBay and only hatched a few or didn't have any hatch at all. As a buyer it can be very frustrating because sometimes it's hard to tell why they didn't hatch, so the first (easiest and less self-blame) thing to say that the eggs were bad in the first place.
I've bought quail eggs on eBay and had poor, decent, and great hatches. I've bought chicken eggs on eBay and only hatched a few or didn't have any hatch at all. As a buyer it can be very frustrating because sometimes it's hard to tell why they didn't hatch, so the first (easiest and less self-blame) thing to say that the eggs were bad in the first place.