I had a 500c that jammed often. It needed a CLA. I had the CLA performed by my local tech, and it came back and worked fine, for about six months. Then the rear shutter started acting up and I had to have a spring replaced. I sent it to David Odess, and he told me it couldn't be repaired. It was one of the early ones, and he said that the parts needed for it were hard to come by and it would require as much to have it repaired as it would to buy another one, in better condition. Around that same time, I also had a shutter on a lens jam. I was told by my local camera repair guy that some of the internal parts had become too badly corroded and he would need to purchase a donor lens to get the spare parts to repair it, and all of that would take a few months and cost too much money.
So I sold all my Hasselblad gear, of which I had a lot. What I found was they were extremely well built, but also very precise instruments. And like anything that relies on that much precision, they don't like to run smoothly if not in top condition. A lot of people crow about how reliable they are, but in my experience, that camera system gave me more trouble than any other I've encountered. I liken them to a Ferrari. They're exceptional when working in top condition, but they're not nearly unbreakable workhorses like a Honda or Toyota (or a more simply designed camera with less moving parts). They require regular maintenance, which isn't cheap. But if you're willing to put forth the time and money, they're really unequaled in their field.
Now, my 500c came from an old professional photographer who made his living off portraits. So it's likely it saw more use and abuse than most. And it's just one person's experience, so you can't gleam much from that. And it's likely that a single jam won't cause any issues. They make a tool especially for dealing with jammed Hasselblads in the field, after all. It's a semi common problem, that's usually rectified quickly and without issue. However, if it jams often like mine and the seller's not being straight with you, it could be a sign or more problems to come.
Point is, you can't really tell from the information you have now if it's a one time issue or a reoccurring one. So if you're worried about getting a lemon, this might be a sign to back away and find another. At the very least, it sounds like the previous owners don't know a whole lot about this camera. These aren't cheap cameras, and while they can be glorious when working properly, they can also be an expensive nightmare (as I found out). So unless you got an amazing deal and are willing to take that risk, it might be wise to look elsewhere. Preferably find one with a recent CLA, as that can be quite costly on those cameras, and it usually costs more to have it done after you purchase it than before. In the end, these aren't budget cameras made for people looking to save money. Their top quality cameras made for people willing to spend top dollar for high end gear. It's like a Rolex, not a Timex. And the cost of owning a Rolex doesn't go away after you purchase it. Not if you plan to keep it for a while.
I'm sure there will be many people who argue with me on this. There always is when it comes to Hasselblads. But it's the truth about what happened to me, and my opinions formed from it.