As the Gringe/Grudge in Kittyland, I have to pipe in, too. I know that people are often very disappointed when they proudly display their breeding efforts at some sim and no one seems to care. This is not because their cats or breeding is worse than others, but due to the fact that there are very very many cats on sale around. The competition is tough and often leads to the sellers underbidding each other.
A good rule of the thumb if to sell a cat at all is to ask yourself "would I pay for this one?" or even better "Would someone, I know, cough up what I am asking for?"
Kay is right about the tendency for highly traited, in numbers and recessiveness, cats having better chances on the market. Especially if the pedigree implies a possible 9T with really rare traits.
The next problem area will be the pricing of your cat. I think it is a misconception that any cat will sell if the price is right (besides being so silly as to sell them around the menagerie value so ppl convert them for K$). Lower prices will not sell a cat but lower the benchmark. (Think of yourself again, when you don't want something, you wouldn't buy it even if cheap, like in a sale)
Only invest as much as you have a rough idea to gain, like how many cats you have a realistic chance of selling. But give your stall some time to establish.
As mentioned before, it won't do to throw out some boxes onto a market stall end expect the money to roll in

make your shop unique. Thus, work on unique cats, this will be some way to achieve your niche in the competition. Better not focus on "What would sell?" but on "What do I like breeding?" This is way more rewarding and trends change very fast. A fascinating combination of traits might make make your signature cat.
Choosing a spot to sell is a difficult decision, too. Market sims have a lot of (even real) traffic, but you have a lot of competition around. Best look for a spot near the landing point or some places of interest like the auction stage, bidboards and other places that are being spammed. When you have your own place, the chance of cat buying footfall is low. You will have to establish that place on your own, but a lot of lag plagued people might prefer to have a look than to have a crashing spree at busier places.
Some final remarks on advertising your treasures: better to yell clear than loud. Tell ppl what you have to offer, but don't do it too often or fill half the screen with ASCI art (a lot of ppl will mute you if they feel disturbed) Aggressive marketing often turns out to be contraproductive - like having some gadget to press your LM or whatever on passers by when they stroll a market sim.
I hope I didn't paint the cat biz in too dark colours and best of luck