HGG Community Forums

I just checked, and it looks like you have 4 horses for sale on your secondary account. I think that would be why.

If you have some horses that you want listed for sale, but have already hit the sales limit, you can list them for sale in chat rooms. Chat sales don't count towards that number. However, that does limit the pool of players that will see those horses, unless you make an ad in the forum.
I like to do that with Rank Specials. I made a chat room specifically for selling them, made an ad about it with a link to the public room, and keep only Rank Special foundations listed in that room. The other advantage is that chat room listings aren't removed at monthly rollover, so you don't have to go through and re-list them every month.
If you don't want to make your own sales room, you can always join public rooms made for certain genes, bone density, etc. and list them there. That also makes sure that your horses are being shown to a group of players that are more likely to be interested in them.

It just didn't pop up with a sale box on the horse's page.

Out of curiosity, why is there a limit on how many horses can be for sale?