Dovewing it looks like when Jered said that they would be sent the 17th that was not true. I have been waiting for my money since the 29th and nothing. Jered has stopped replying to emails now.
I'll give you a hint- rarity!!! And quality is also a huge factor. Crystal cores tend to bring more money, and obviously the more color you see in the opal the better. Also it is hard to judge by a picture so know your dealer, and know the return policy!
I know it's a lousy answer but the more of them you own the more you will understand. Reading about stones helps as does seeing them in person, like at gem shows.
I just listed a couple of new ones Binox- another synthetic star (but this time a blue sapphire) and also a Mali garnet. I'm with you- I am ready for bids to pick up!