I see many people who post their pets for sale, breeding, etc. How does one determine the value/worth of the pet? I'm not sure if I have any pets that have a gene that is valued or anything, and it would be nice when I am out exploring to know what pets/qualities I should be keeping an eye out for.

Pet value is pretty subjective from person to person, but more carried and visible genes a pet has, the more they are usually worth. Newer genes are the most popular for the most part. :) For example, the new Vulnyx genes will be your best bet.

There aren't any fixed values, most of us just go by what we see. If you spend enough time in the forums you'll quickly figure out which species and genes/mutations are highest in demand. Like Deathdust said, newly released g/m tend to be the most sought after - same goes with species, btw. I'd tentatively include vulnyx, as their recent change in lineart means plenty of people who didn't bother with them before are going to want a few for their collections.

I wouldn't advise trying to make a profit by capturing wild sylestis; that market is practically nonexistent. You'd be better off saving your nets for the summer festival.. tagged event pets, especially the more popular ones, can go for ridiculous amounts.

It's not just about genes, mutations and species...it's about COLOR, about BEAUTY. I'd pay more for a pet with fabulous colors and one gene or mutation than I would for a washed out, drab looking multi-visible. Above all, breed for beauty.