Demand varies from player to player so we can't really give you exact values for demand, it all depends on how much you're willing to take and pay for a pet.
I personally see the dark leopard going for fair trades or minor underpay. Some people might overpay if they really like the pet, that's just what I usually see. It's not too hard to trade it away, specially if you're trading up the list.
I see the raven usually going for major overpay, some people might trade fairly or underpay for it. Anything is possible!
The CS community is huge, I'm sure you will eventually find someone valuing those pets at exactly the same as you do, there is no need to adjust to the "demand value" that other people want.
Rarity wise the dark leopard is worth around 9 mid advents and the raven is worth a tier 7 advent