I have a ton of the sugar combs, and I had someone one give the a Great Escape as a gift. The basic difference is that the Great Escape is bigger than the regular size, and they use different hardware. And of course the higher price. I ended up exchanging the one I got as a gift because I didn't like the color. There are similar items sold for rats too. (Triple hammock)
The problem with having a exclusive design, even if you do patent it, is all someone needs to do is make a minor change and call it something different, and it is considered a different item. Why do you think you see so many knockoffs, and generic items that are almost the same as a name brand item.
I think it would be nice to just write a short note, and let the person know what you're doing as a courtesy.
If you're thinking about protecting your own design, you really can't. As you've found out the patent process is expensive, and can be slow. And the cost of taking someone to court would be high. And not everyone that would sell on the internet would care about the ethics of copying your design.
When I was young, and I wanted to copy a crochet design I saw at a craft show I would actually buy one of the item from the person, so that they actually did get some business from me. I wasn't selling the items either.
Just realize from the start if you post an original design, it will be copied. Many people that sell on ebay and etsy are not part of the community, and even many who are would not have a problem copying your design, and selling it.