I'm not a lawyer or anything, but I think the use of muppet / muppet style in your ebay title would not get you in trouble by the big D. If you search around on the internet for puppets you'll see that a lot of companies also use the words muppet and sesame street to describe their own puppets for sale.
In my eyes, a muppet style puppet has to look somewhat similar to a real muppet puppet. And we know from Avenue Q and other places that you won't get in trouble if you build and perform a puppet in the style of the muppets.
You can have a vent puppet (like a furry rabbit) that doesn't look anything like a muppet puppet, but it still has moving mouth and other features.
Once you start using ping pong balls and spoons for eyes, a big round nose, antron fleece and other muppety features . . . then it turns from a regular vent puppet into a muppet style puppet? At least that's my take. If it looks like a muppet, it's a muppet style puppet!