I have one such story, but unfortunately the battle is still going on and I don’t have the card in my possession yet. As you all know, I collect Pikachus. One of the cards I’m still missing is the Italian HG&SS Pikachu with Pokémon Day stamp. At the end of last year, after collecting Pikachus for about three years, one finally popped up on eBay for bidding. I was able to win it for around 20 USD, paid the seller, and it was shipped out within a few days. One week later another one popped up (of course… happens pretty often tbh. You look for a card for a few years, and then two pop up in the same month.) Anyway, since I already had mine incoming I ignored it, even though it was like 12 USD BIN.
Fast forward about five weeks, and my card still hadn’t arrived. I contacted the seller, but no response at all. In the mean time that other card was of course already sold. A week later I send another message to the seller. Again no response. I figured I’d wait one or two days and if I’d still didn’t had any response I would ask my money back through PayPal. Unfortunately I was a bit busy with work at the time, so I forgot, and by the time I remembered the 45 days time to get a refund through PayPal were overdue by a few days, so I couldn’t get my money back… 
Fast forward by halve a year later, a new one popped up for about 15 USD BIN. I was happy to find one again, so bought it, and the seller shipped it out that same day. Fast forward about three weeks again, I started to get worried again, so I contacted the seller if they knew the whereabouts of the card. At this time I already checked whether it was the same seller as before, but it was someone else fortunately. This seller also kindly replied to my messages, and asked if I could be patient for one more week. If it then still hadn’t arrived he would refund me. One week later still nothing, so I contacted the seller, and he indeed refunded me as promised.
I really have the feeling this card is cursed for me based on this above… I already made a deal with a friend that iff a new one pops up I’d let him buy it for me, and then he can send it to me… Ironically enough this means more can go wrong since it first has to ship from the seller to him, and then from him to me, but whatever. 
I just hope it will one day actually arrive after I purchased it…
Another example I could give, which did arrive eventually, are Portuguese cards in general. About a year ago I only had like 15% of all Portuguese Pikachus and Sevipers that exist in my possession and it was the hardest language for me to find. Most of the Portuguese Pikachus are produced in Brazil (except for the first few sets, which were also released in Portugal), and since I live in Europe (The Netherlands), those are pretty far away from me, and not as easily accessible through the internet as cards from the US, Australia or Japan.
So in my search for a middleman from Brazil, I was first contacted by someone on Facebook. I’m not sure anymore how he found me, but at first he wanted to help. Later on he said he was busy with some things, and eventually he stopped responding to my messages all-together…
A few months later I was contacted again by someone on Facebook. He had seen my YouTube collection video where I briefly mentioned I was still looking for a middleman from Brazil. And he was able to help me A LOT! In just two weeks time he was able to find 13 of the 19 Pikachus I was missing. And the week after he was able to find 6 more (including one I didn’t even knew existed), so there was only one missing. It was also the first online trade I did, because he collected Pikachus himself (one of each artwork), and he was still missing most of the Jumbo Pikachus. So I bought a few Jumbo Pikachus from eBay that wouldn’t send to Brazil, and after all had arrived, I send him the Jumbos and he send me the Portuguese Pikachus and Sevipers (plus a bit of money he insisted on paying, because the Jumbos were more expensive in total). Here is what I posted about that when the cards had arrived on April 20th, 2017. After that he was also able to help me with any future releases, and he still helps me now every time a new Pikachu/Seviper/Mimikyu is release in Brazil. He also helped at least five members here on the forum from what I know. Truly an excellent middleman that helped me a lot. 
Greetz,
Quuador
