To also put this in further perspective. You block someone, because you do not want to interact with them. For whatever reason, which is fine. You both still have access to the same forums, and the same features, you just don't see them, which is what works. They shouldn't be able to message you, or you be able to message them, because that is an interaction. I would love for the block button to be able to hide their posts on the forums, like how Discord does it. Those are means of interacting with someone. Buying a wolf from the Flea Market for a set price, that the seller sets, is not an interaction, since no words are exchanged, or can be exchanged. I think this is fine, since the seller still benefits from being able to sell the item/wolf/etc, and the buyer gets what they wants, they just can't bargain with the seller. The Flea Market is a public part of the site, that is just a means to exchange wolves, and items.
It would also be extremely unfair for a user to block someone, and still be able to buy the blocked person's items, and wolves, while the blocked person cannot buy the items from the person who blocked them. That could also lead to harassment in the long run, and lead to an unhealthier Flea Market. Overall, just no support for such a feature. If you're that concerned about a blocked user buying from you, use the forums instead.