Essentially, at this end of the scale, it's not a trade for someone who wants instant gratification. Be prepared to get yourself known - this is one of the few face-to-face trading scenarios left in the game - and make yourself available via IM or the like. It can be very rewarding building a clientele, and in this aspect it's more a merchant skill set than a taming one.
If you don't enjoy it, you're seriously better off doing something more profitable like farming gold, items, or scrolls.
This is excellent advice, as long as things haven't changed drastically in the year since I left. What you want to do is become known. I had people sending me ICQ's and wanting to buy pets for a long time after I left. I think my last request came only a month or two ago. But I was known for selling extremely high quality pets.
I made a lot of gold selling pets. I'm thinking over 200mil in the two years or so that I was a serious pet merchant, but I probably could have made more gold in the same amount of time, other ways. But it was fun, and I obviously kept the best of the best that I found.
I'll let you in on a little secret. This may not seem obvious, but the best market for high end pets will be among the "hard core" pvp crowd. They are the ones farming high end content for scrolls, or artifacts, and they are the ones that have the gold to spend. It is nothing for a pvp'r to spend 20 mil on something special.
But if you are going to become known for selling high end pets, you had better make sure you gain the required skills, and put in the necessary effort. I have a spreadsheet with the stats of all the major tameables on it, and a system for assessing them that is much better than the pet calculator (although it is similar).
And another little secret, it is just as important to understand your clientele as it is to know the maximums and minimums for pet stats, and what colours are rare. There are certain stats that I might not have thought were all that important, but I knew that pets with those stats were in demand, so I always kept an eye out for pets with those stats.
I'm not going to get into an argument with a customer about whether or not their favourite number has any meaning or not. The customer is always right. But at the same time, you do sometimes have to play the professor, and be ready to explain why some pet that you may think is special, is indeed special.
But lastly, and most importantly, your most valuable asset is your reputation. Just as important as your reputation for knowing your trade, is your reputation for honesty. If someone asks for an opinion on a pet that someone else is selling, make sure you give an honest one, even if it means you lose a sale because the pet they are selling is better than one you are trying to sell.
Nothing is more valuable to as a merchant than your reputation, absolutely nothing. And this is especially true in UO these days, because UO is very much like a small country town, everyone knows everyone else. If you get a reputation for being a scammer, you are doomed as a merchant.