Well, if the seller was selling on evilbay you 
could be malicious and report him. But since you 
have no actual proof that the seller closed the 
auction(s) early, so they specifically could sell 
it directly to a private collector, they might 
not help you. Also, since you already emailed the 
seller and they replied with a pretty vague 
answer, it seems this individual just doesn't get 
it. Furthermore, since the seller is pretty 
upfront about keeping the backgrounds, dougas and 
layouts, unless this doesn't bother you, perhaps 
you should reconsider and avoid dealing with him 
entirely. Someone people don't appreciate and/or 
care of dealers/collectors who do this, I happen 
to feel this way, but since there is no 
enforceable laws preventing this, you either 
accept it or don't.

I didn't see the auction you are referring to, 
but generally speaking, I think it fine for the 
seller to close an auction early. Most sellers 
want to keep expenses down, whether or not they 
use a reserve price (which costs more) is besides 
the point. For sellers using evilbay auction 
costs are not cheap. All of the costs for basic
features, such as the starting cost (minimium 
bid), duration (beyond 7 add more $), and closing 
costs add up. Extras like having a reserve, 
feature auction and more pictures all cost more, 
eating into your profits as a seller. Ultimately 
IMO it makes sense to close the auction early IF 
you get a good price for the item(s).