If you think it isnt the tubing, and what you expected then thats enough to return, and ebay will support. Its a simple open the return with not as described task.
Personally I would speak to the seller first and explain what and why and see what they say. They could be accomodating and avoid the return. I have sold low value things and cocked up the description and when contacted just refunded and said keep the item as an apology. I wouldnt do that for anything more than a few quid though.
If they are a nob about it crack on with the return, but at least you have been reasonable to start with.