I found out someone was trying to sell their character, for which I had drawn the character reference sheet, saying that "You would Get the Rights to All the Art".

No, no you would not.

It's in my TOS, but sadly this bears repeating, I reserve all the rights of work commissioned from me. So do most other artists, as per Western European and American freelance laws, the freelancer retains the rights to the commissioned work unless agreed on otherwise. Meaning, the client has to state what rights they need, they pay for it and they get it in black and white. A contract protects both the freelancer and the client.
And furry commissions are freelance, not "work for hire", like some people think. "Work for hire" involves having the artist sign a work for hire contract which transfers all rights to the employer, but entitles the artist to a regular paycheck and benefits (otherwise, why sign away all your rights?)

If you'd want to sell the rights to a drawing you commissioned, such as selling the art as part of a character design sale that would entitle the new buyer to repost/edit/etc. the work, you'd have to own those rights first AND commercial rights to sell them AND the right to transfer those rights to someone else. This means you'd have to ask for those rights from the artist.

You don't own the rights to artwork just because your character is in it.

"Gosh." you might think "Why are you telling us all this?"

To try and prevent disappointment. As of this moment I have not transferred any rights to my artwork to anyone (save for some unrelated logo design work).
I don't mind people reposting their own commissions to some pages, or cropping for icons and whatnot, but I feel it's important to state: You cannot buy (rights to) my work from anyone but me!

If you buy an adopt/character design from someone, and they said past commissions are included, unless they themselves are the artist or can show you a contract for the artwork, they're probably not in a position to transfer any rights to the work to you, and you might not be able to use that artwork. So if you buy an adoptable with an existing library of artwork, only pay what you would pay for the adoptable/character itself and not the extra art. You'd be potentially throwing away money.