I'd recommend that you take a trip into New Magincia and have a look at the pet vendors there. That will give you a good idea of what others are charging and the kinds of pets they are selling. The market is much the same as always - players want the strongest pets they can get, some want to buy them fully trained too. Greater dragons are the "in" pet right now, but you'll find many other tamers trying to sell them. So it's a choice between trying to find a bit of that market to sell in or perhaps tackling something different. Don't be afraid to try selling something the others don't stock, if it doesn't sell you can always release it.
Also, if you're going to tame some pets it does help if you are taming and selling pets that you enjoy working with. I love the cu sidhe spawn for instance, my bard tamer got super gains down there and it was nicely profitable. It's quite a relaxing spawn but the odd paragon changeling etc keeps you on your toes. Then I'll go hop round different places and tame whatever I find. It's good fun if you've had a break from the game.
When I'm trading, I like to sell a range of different pets, from coloured ones like the cu and hiryu to rune beetles and the like. Then I get a nice variety of pets to suit different tamers and you don't get bored building up your stock.
I try and sell my pets for a fair price rather than "as much as I can get" so my vendor isn't full of unsold stock for long periods. Or you can still do a fair bit of selling in person or via the forums too.
Wenchy