Colony animals don't appear on the trade list (either greyed out or otherwise) when the outdoor temperature is outside the animals' comfortable range, even though they appear on the trader's "want to buy" list.
For me, this occurred during an ice sheet run with the outside temperature at -60c, which is below the minimum comfortable temperature for my colony polar bears (-55c).
I've seen a case by another player of the opposite occurring: chinchillas not appearing on the trader list when the outside temperature was greater than their maximum comfy temperature (https://www.reddit.com/r/RimWorld/comments/9v3ads/animals_arent_appearing_in_orbital_trade_list/).
Digging this out but it this one has also caused me a lot of frustration. Please fix this, it doesn't matter for orbital buyer what temperature is on the ground. The lack of information caused me to think my save was broken and I could not find this information anywhere until I read some post on this forum by accident.
I can confirm that this still happens in 1.2.2900. No mods, no Royalty, no Steam, Linux version.
Spawn and tame animals, summon trade ships, confirm that animals are salable, throw some cold snaps to make temperature -60°C or so, animals unsalable.
Cancel cold snaps, heat wave to moderate temperature, animals salable again.
Heat wave to +60°C, animals unsalable again.
Shouldn't matter since they get beamed up to the orbiting interplanetary tradeship.
Confirmed this bug and I have it down, thanks!