ex wives
Yeah, we used to go camping with some neighbor families when the kids were younger, and all of them went thru the camper-trailer phase. Getting a big trailer necessitated also getting a big vehicle to pull it as well. One year several of the families all had new trailers and new rigs to tow it (at substantial cost, I am sure), and the next year, in typical keep-up-with-the-Joneses style, two of them traded-in their first trailer for an even bigger one, after one season. I am sure this was costly. By the 3rd or 4th season after the initial rush to get trailers, no one was camping any more, and the trailers and big pick-up trucks disappeared from the neighborhood.
Except us, because we bought a used pop-up tent trailer for a song that could be stored in my garage at home for free, and could be towed by one of my existing (and paid-for) vehicles. We still did family camping a couple times per summer for a few more years, but eventually family tastes changed and we stopped going as well, so I sold the tent-trailer. 
Happily, I got quite a bit more than I paid for it, so it was like we used it all those years for free. But, it did sit, unused, for most of the time, like most other trailers and RVs.
Oh, God don’t get me started on Mr.Wrekkers hunting, fishing and camping equipment. Sans guns, he has a large amount of money tied up in: ATVs, tents, a camper, 2 boats, portable deer blinds, climbing tree stands. And various and sundry smalls. He does hunt and fish, ALOT. There is no way he’ll wear any of it out before those days end.
He even has a special metal building just to store the junk in. A veritable warehouse.
Apparently some people buy clothes they hardly ever wear.
I have a Jacuzzi bathtub. I do use it as a bathtub regularly, but the jacuzzi jets only got used once or twice while bathing. You’re supposed to run them every couple of weeks with a bathtub full of soap just to clean out the jets, but that hasn’t happened in years because the motor stopped working.
This is the large purchase I regularly regret.
FWIW I have a quality treadmill that I bought 3 years ago that still gets regular use.
On the other hand, the router (woodworking tool type) and router table I had to have several years ago has been used exactly zero times.
mmm
I bought one of those clip-on macro/wide angle lenses for my Pixel phone. It works okay, but it’s too much of a hassle to use for quick photos. I’m giving it to my brother…I doubt he’ll use it either. Maybe he can find someone else to give it to.
I’d be willing to bet it’s right around 50% of the population. 
Guitars. A lot of people like the idea of pulling out the guitar and banging out a song but figure out pretty quick that it takes a lot of time and effort to get good enough to play in front of others or jam with other players. Same goes for other musical instruments I’d expect but I have more insight into the used guitar market.
Motorcycles. Many people get visions of all the fun they would have on a motorcycle but soon come to realize how much planning has to go into the average motorcycle ride as far as dealing with weather. At least here in this part of the country. Can’t count how many times I’ve taken off on a ride in hot sunshine only to run into a cold/rain front. Unless you take all weather gear with you it can end in a long wet ride home. I’ve known several people that bought motorcycles, put a few hundred miles on them and then sold them. I’ve had my current one 25+ years, the one before that for 12 years.
That’s hardly limited to board games. I dropped several hundred dollars buying two of the Warhammer 40K Ork Great Gargants (produced by Armourcast). I never played a single game with them, never even painted them up. I wonder if I can get some of my cash back through eBay.
We got a pop-up camper when I was a kid, and probably would have used it more if my mother hadn’t hated camping so much. For most of the next decade, it sat unused in my grandmother’s outbuilding, and then they finally sold it.
The sleeping bags did come in handy when my sister and I went to Girl Scout camp, or my brother with Indian Guides.
I’m pretty good about not buying too many clothes. I hate clothes shopping. In fact I hate clothes wearing. It just so much of a hassle. They have to fit, they have to match and be appropriate for the weather. It’s just stoooopid! I hate shoes worse. (Don’t say it, not all Arkansans go barefoot).
ETA the lil’wrekker buys plenty of un-needed clothing to cancel me out.
A tiple. Don’t know what I was thinking at the time. Never could make heads or tails of the damn thing. Fortunately, I found someone to take it off my hands
I have a Starship Enterprise pizza cutter that I’ve never used to cut pizza. But then, I never really planned to use it. It’s just cool.
We’ve got a Starship Enterprise telephone that someone gave us. It’s never been used.
That is cool!
Our biggest is a set of sterling dinnerware that we never use. We put it on our wedding list 54 years ago and wound up with a service for 12, but my says that if we use it it will tarnish so we never do. I am sure that cost the equivalent of at least $10,000 today.
We have a Foreman grill that was used quite a lot when new, but now not for several years. We have a waffle iron that we used a few times, but i prefer pancakes. I have a pasta machine that I used to use regularly but haven’t for a couple decades. We have a breadmaker: me, and I make bread at least every week to ten days. My wife lobbied for a mixer and I finally yielded, but she never uses it (although I have started using the dough hook for bread). About 40 years ago, when I was heavily into woodworking, I bought a router/planar. But I had to get a motor and hook it up. I never did and got into computing instead. Finally, although I am not sure this counts. I bought a new car nearly 12 years ago and it has less than 20,000 miles on it. Use it maybe once a week.
Yeah. I live in a perfect spot for a telescope. Way, high altitude, far from any city lights. Well, even in the Summer, it’s cold just sitting around outside without a fire. And I really don’t have the patience for it.
I do use my bread machine. Also use it for making fresh spaghetti noodles, pizza dough and pretzels. It’s SO much easier than doing it by hand.
Gym memberships. It’s part of their business model - they’d go broke if every paying member went regularly.
People buy a lot of kitchen stuff that they only use once - fondue sets, chocolate fountains, etc. So far our immersion blender has been used an average of once a year. Even less for our bamboo steamers.
The thing is I actually play a lot of board games. I usually get together to play a couple of times each week. I checked my BGG log and I played 218 games in 2018.
So my problem isn’t under-using; it’s over-buying.