As for my worst deal: buying the third YOU Wagon Pikachu that was put up for sale this year. The Pikachu YOU Wagon promo was released a couple of years ago, and I hadnāt seen a single one for sale since I started collecting Pikachus in October 2015. About a month ago one popped up on the Japan Yahoo, and it ended at 135,000 Japanese Yen (~1,175 USD), which I lost (my max bid was 105,000 btw).
After that first sale, another one popped up which was sold for 95,000 buy-out within two hours before I even saw it; and the third one started at 1 for which I placed a bid of 77,777. That same week 5-10 more auctions for this card popped up, varying from 65,000-100,000. All due to that first high sold price.
I eventually won that third one for 77,000 (just 777 below my max bidā¦). I would have preferred to be outbid, because there were some for 65,000 buy-out available, but āunfortunately I wonā (and no, I couldnāt cancel my bid at the Middleman website I used). Now there are still a couple for sale, with the current price varying at around 30,000-45,000, which is less than halve of what I paid for mine. The lowest that is currently for sale is 200 USD on eBay (bidding however, not buy-out). Needless to say, I should have waited just like with those pre-orders. In just one month time I could have saved around 500 USD.
Having said all that, Iām still happy to have it in my collection, and I havenāt regret buying it. Yes, the price is still lowering, and I could have saved a lot of money, but what if the opposite was true? What if that third one I placed a bid on would have been the last, and I wouldnāt have won it? Itās a beautiful card, and at first I was already kinda pissed that the first one went this high causing me to loose. Now Iām just glad to have it, and I left the pricing of the ones that are for sale as we speak behind me.
My motto in my hobbies is: āI only have to spend the money once, and then I can enjoy the item for the rest of my life.ā Well, thatās exactly what I did. 
Btw, this is the card Iām taking about for anyone not familiar with it:
Here is an article with some more information regarding itās release at the end of 2014.
