No, he doesn’t have to be a certain level. If he’s a puppy, you have to wait. You can view his status under Bloodline to the right of your wolf. Or if he has recently been bred, there is a cooldown period which you can also view. Also if he has a companion or apparel attached, you must remove them before you’re able to sell. Hope this helps!
As far as I know it shouldn't matter unless they have Apparel or a companion. I put pups for sale all the time I'm unsure if being on a breeding cooldown affects it never tried
did you accidentally select (for stud) instead of (for sale)? when you
select (for sale), every single wolf shows up, even those you can't sell
because they have apparel or companions, and breeding cooldown doesn't
affect whether you can sell a wolf anyway. when you select (for stud),
only adult male wolves that are able to be bred appear
It might also be the order the wolves are shown in that is throwing you for a loop. From what I understood from someone else, the order in the Flea Market makes absolutely no sense. It's not alphabetical (names), it's not based on ID/age, nor is it the order of the wolves in your den. Instead it takes the first wolf of every den, and then the second wolf of every den, and then the third wolf, etc. etc. etc. So if you want to sell a specific wolf, it may be worth it to move it to the first slot of your first den to make it easier to find in the weird way it is listed (and only 6 at a time. I have nearly 500 wolves. Devs, please!)