You should have bought 200 and resold them at a profit using craigslist.

No problem at all!  Retailers must sell it at the price it’s marked.  Years ago, my Dad discovered an entire bin of jumbo sized rain umbrellas marked $1 apiece or $2.50 for three.  He came out with 15 umbrellas for $13 including sales tax.  They were given as Christmas gifts to 14 people!  He kept one.  I don’t remember for sure if it was K-Mart or Ames that had them on clearance.  I still have mine!  I believe it was in the early 2000’s.
Yes, he could have sold them, but there weren’t as many venues at that time.  

OP here. I’ve no doubt that the deal itself was morally acceptable, I suppose what *slightly* bothers me is the fact that I feel great satisfaction that they lost money on the deal.

That's justice you feel, think of all of the businesses they've screwed over the years.

All these places are no good.  

I actually have a bigger doubt for "use their return policy to my great advantage," not sure what you meant by that but I'm very tempted to return little things I bought which they would usually refund without asking question. But I feel like this is such a Jєωιѕн trick. I actually learned it from a real Jєω and I felt immediately disgusted by this way of doing things.

Don't. 

I am constantly buying things on Amazon, using them for a few months and when they don't work well anymore I order another and return the old one. 

If they're going to sell cheap China junk, then they are going to get dinged.