Hello!
I discovered today that you are able to list wolves wearing Custom Apparel for sale onto the Flea Market.
With some testing, I had another user purchase said wolf and it went to their den like this:
As you can see, there seems to be a visual glitch where the wolf retains it's Custom Apparel Image, however, the wolf is no longer actually wearing it (see Apparel section).
The Custom Apparel remained in my own Inventory.
This visual glitch was viewable by me, the buyer, as well as outsider viewers.
Upon returning to my den, the Custom Apparel was automatically re-applied to my wolf without my taking any action.