 Re: is it possible to hide sold items on ebay?
There isn't a way that I know of to hide the number of items sold on a listing.
You can use sell similar, but all your history and 'sales velocity', your position on the search, etc. will be gone.
The feature he referred to in selling preferences is self-explanatory and is not used to hide the number of items sold on a listing. That's probably why neither of you could get it to work.
What's alarming is that the uploader is a new eBayer who's actually selling an e-commerce course. Unless a person has trained himself for his chance, the chance will only make him look ridiculous 
I'm joking.
Anyway, I'd surmise you're asking because you wish to prevent competitors from copying your successful products.
The bad news is that this isn't always easy.
One thing you can do is have a brand with intellectual property rights; i.e. trade secrets, copyrights, patents or trademarks - depending on what it is you're selling. That's a lot of $$$$
Another option is an exclusive distribution license. This may cost you less $$$, but it might be harder to find something that is both in demand and profitable at the same time.
If any of the options above aren't within your reach, you may consider dropping items that have saturated the market or no longer profitable and move on to something new. Your inventory might be in a constant state of change.
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