This isn't a knock against greater dragons, I love them. I think they helped put the tamer template back on par with the other popular templates. Having said that, the only pet that I know of right now that I would be able to sell easily, would be a truly exceptional greater dragon, and even that I might have a hard time selling.
The truth is that greater dragons are the flavour of the day, with good reason, and they turned out to be a lot easier to farm, and it has turned out to be much easier to find really good ones than I expected it would be, so even for greater dragons, the only one that I feel I could probably easily sell would be a truly exceptional one, and even at that, I might have to go to the trouble of fully training her before I was able to sell her easily for a good price.
A pure white untrained cu sidhe, before greater dragons, with decent stats, should easily have fetched 2 mil. Now, I have no idea, and I suspect it won't be very easy to sell her period. I would probably suggest trying to sell her for one mil.
And I shouldn't blame the whole collapse of the market on greater dragons either. Part of the problem is that gold is much harder to come by than it used to be, and everything has dropped in price, significantly. The developers should be happy about this, because I think this was what they were trying to do, so they deserve some congratulations for this.
The other factor is that I think a fairly large number of the veteran, more wealthy players, have been leaving, and at the same time, if the publicity is to be believed, there have been a lot of new, and returning players, which is terrific, but it is a two edged sword for merchants. New players need more stuff, and are a good potential market, but they don't have access to the gold that veteran players have, especially for higher end merchandise.
There is also one other, somewhat depressing factor, and that is the overall state of the rl world economy. Times are tough right now, and people don't have as much real disposable income to spend on luxuries, and that includes virtual gold for a game like UO.
I guess my final verdict is, this isn't the best time to be a high end pet merchant. One thing you should always keep yourself open to is trades. People who don't have enough gold to buy what you are selling, will often offer killer trades. So don't be fixated on gold, you can pick yourself up some very nice items that you might have a hard time finding any other way if you are willing to make trades. Make it clear you are willing to do this.
I guess my final piece of advice might be, if you are really looking to make some gold as a merchant, lower your expectations. Now might be a good time to get into selling pets that are less expensive. Blue pack beetles are always in demand, and you should be able to sell them easily for 20k each. I'm not sure you would be able to make gold faster than just going to your favourite hunting spots but if you really want to be a pet merchant, then this is something you might want to try.
I have some other ideas as well, but I'm not ready to share
all my secrets yet.