Some bad deals were paying for pre-orders of a soon-to-be-released card, and then having the prices drop to less than halve three weeks after it has been released. Had this a couple of times, so now I won’t pre-order anymore, and depending on the amount that is being sold on eBay I decide whether to buy it in the week of release, or a couple of weeks afterwards after the price has dropped. These differences are usually 40 → 20 or 18 → 5 USD or something similar, so nothing too crazy.
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 hadn’t 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 bid…). I would have preferred to be outbid, because there were some for 65,000 buy-out available, but ‘unfortunately I won’ (and no, I couldn’t 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, I’m still happy to have it in my collection, and I haven’t 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 wouldn’t have won it? It’s a beautiful card, and at first I was already kinda pissed that the first one went this high causing me to loose. Now I’m 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: “I only have to spend the money once, and then I can enjoy the item for the rest of my life.” Well, that’s exactly what I did. 
Btw, this is the card I’m taking about for anyone not familiar with it:
Here is an article with some more information regarding it’s release at the end of 2014.
Oh, one other thing that was a pretty bad deal… I came across a ‘Thin stamp’ Base Pikachu. Or so I thought. I was in a bidding war with two other people, and eventually won it for little over 100 USD, with an additional 50 USD shipping from the US to me. I was very happy to finally come across one, and even more happy I won it.
When I received it everything seemed fine, and I was still happy with it. Only the day after when I took another look at it I noticed something odd: it wasn’t Shadowless!.. Although the card looked very real, after doing the light-test I indeed confirmed my fear: it was a fake card…
I did get the money back for which I won the auction (after about two months and in two separate refund-sections…), but I did loose on the 50 USD shipping… Ah well, I did learn to double-check every image before making a purchase since then. And unfortunately I’m still looking for a Thin stamp Base Pikachu…
Greetz,
Quuador