Can’t really think of anything I bought way below market value (the other way around twice though 
), but there are definitely some amazing purchases for me for other reasons.
The best purchase is probably an impulse-buy in August of 2015: I purchased every Pikachu card that was in stock (and available for less than 20 USD per card) that I didn’t had already. In total about 50 Pikachus or so. Before that I mainly collected Yu-Gi-Oh cards, which was also the reason I was browsing TrollAndToad.com at the time. Before they arrived I already decided I would start a Pikachu collection. I looked up which all existed on Bulbapedia, and also came across some foreign websites to buy cards in various languages.
I think I had around 250 unique Pikachu cards when I came across Justinator’s collection thread and website. At the time he had around 550 Pikachus and I was really inspired! He even had loads of cards I had never even heard about in the four months I was collecting Pikachus. Quite nostalgic to now look back at my first comment on the E4 forum in Justin’s collection thread. 
That impulse purchase alone is the reason I’m now here and collecting Pokémon cards again. And I must say that I still deeply enjoy it. I now have more than 1100 unique Pikachu cards and have been the top Pikachu collector of the world for about halve a year (now I’m second after Tom from Belgium). The community here on the forum is also one of the best I’ve been in, and it’s still a blast to read all the new threads and comments that people made on a daily basis.
Anyway, back on topic a bit. Three other noticeable purchases I can think of:
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The MATCHPRINT Pikachu. Only two copies exist of this card in the world. Although the price of 1,000 USD may seem big, considering there are loads of Pikachu collectors out there and only two existing copies of this card, I’d say it’s a very reasonable price and I’m very glad I had the opportunity to buy it when I did. Still one of my favorite cards in my collection.
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I recently (last month) won an auction for a Chinese EX Legend Maker box. Won it slightly below market value (111.11 USD - market value is about 150-175 I think), but with the additional 20 USD shipping and 35 euros import fees it was still about that market value.
Still, the cards I pulled from the box were totally worth it. Not only did I pull the hardest to pull Ultra Rare (Flygon EX), but also the Gold Star Secret Rare that I wanted most from the set (Registeel). Here my recent post about the pulls in my collection thread. Still waiting for some Moltres to arrive, but will soon send the Registeel and Flygon to BGS to grade them. -
Won an auction of about 7,000 Japanese non-holo cards on YJ 6-8 months ago. Although bulk sells like crap these days, as a collector I’m glad I won this auction. From the total 7,000 cards about 2,750 were new for my own collection. The others I have to sell one day. But, considering I only paid 100 USD for the auction (plus about 70 more for shipping, middleman fees, and import fees), this means I only paid 17000/7000 ≈ 2.4 cents per card. Since it contains at least 200 cards that are selling for 2-5 USD per card, I could easily triple the money I spend with the duplicates alone, and still keep all the other cards in my own collection. And it also contained a lot of Pokémon I didn’t had yet, so recently I bought the remaining Pokémon I was still missing (mainly Mega Pokémon and other forms) and I now have the National Index including all those forms complete.
Greetz,
Quuador