To each their own, but this feels like a lot of fuss for no reason...or worse, a case of entitlement. This is another reason why I avoid Ebay selling like the plague these days. The seller was absolutely wrong in his description of the item and if your intent was to keep it sealed then I would back your actions 100%. However, your intent was to open the item anyway. You had added hassle in getting the hair piece, but got it for free and you clearly didn't feel it was worth getting the full refund to go buy from someone else. The desire to get an additional discount based on someone's mistaken description when you essentially got what you wanted doesn't sit well with me.
Shouldn't matter what I wanted to do with it OR that I got the replacement part for free. And yeah - if I bid on an item that is advertised as mint/unopened and it doesn't come that way - then I'm entitled for it to arrive that way. I could have decided to keep it MIB after I received it and then buy it brick-for-brick on Brick Owl instead - that option wasn't available to me, the seller saw to that.
Seller could have just as easily gone on their own, inventoried the set, gotten the missing pieces (like I mentioned earlier, I'm still not sure I have all the parts since Lego ALWAYS puts extras of the little pieces) and then sold it as "Previously Used" - and then guess what? I wouldn't have bid on it - and since I was the only one who did, I'm doubting anyone else would have either. The seller would have eventually had to settle on either keeping the set or selling it for a much lower amount. Plain and simple, that's not a sense of entitlement or looking to get an additional discount, it's how the after-market on Legos works on eBay - mint/unopened sets command a premium, opened/used sets get at most the original MSRP. All it took was for me to look through the last couple weeks of completed Lego auctions to see that.