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You may see some horns of interest under "Mellophone" which are obviously not mellophones.
Avoid these if the seller has under 99% rating.

No Ebay scandal here, just general advice you already know.

I cannot imagine why anyone on a tuba forum (especially me) who might be interested in searching Ebay for mellophones.

Since I guess I have too much time on my hands, I looked at 5 pages of search results for "mellophone", and noticed one instrument that indisputably isn't, some kind of pocket trumpet.

But in my opinion the word isn't really well defined. If you have some stricter definition, then you're welcome to stick with it, but it seems to me you can't expect everyone to already be familiar with your definition.

I said it wasn't actually a mellophone. That's just the MO of a certain New Jersey-based dirtbag.