Gems on the market here tend to vary anywhere between $1:550g to $1:800g; I don't usually see anyone selling treasure for USD, but on the site itself treasure ratio fluctuates between 1g = 1000-1300t right now.
I personally think gem amounts above 25,000 (which is about $31 to $45 depending on what ratio you're using) are generally a decent amount and worth selling, and that's around the amount I've seen most customers interested in as well.
As for familiars, some are quite expensive!
The familiars and apparel from 2013-2014 in this post as generally sought after, especially the Light Sprite & Sunchaser Jewelry: (you need an account to see links)
Other familiars worth noting are downtime familiars (Speedy, Red-Footed Akirbeak, Golden Bantam Fanger), any Kickstarter familiar (Skycat, Golden Idol, Bone Fiend, Cog Frog), and the Beta familiar (Boolean) if you have them.
Some dragons are also worth a fair bit as well - these are mostly G1s (short for First Generations, which means they are a dragon that has no parents or offspring listed on their page) but their prices entirely depend on if their colourscheme is desirable.