Same as anywhere, you get some good sellers and some bad. If you check the sellers feedback before buying, it will give you some indication as to whether their eggs are well packed or fertile. If all goes wrong you have ebays buyer protection to fall back on.
I recently purchased 20 eggs on ebay and 18 made it to lock down and 14 hatched. I was very happy! I spent a lot of time reading pages and pages of reviews before I purchased. I would highly recommend you do that too.
So far I have only bought eggs off ebay. I bought from two different sellers and the eggs arrived on the same day, so in they went. Out of 20 eggs I got 6 chicks.