I've talked about knowing in advance who you think will buy a product before purchasing it, perhaps having entire groups in mind. Lately, there has been another idea that has floated around, it's "That guy." That guy is the final person to buy a game before you decide to drop it.
For board games and role-playing games, there may be only one purchase, even within the thresholds you would normally re-order. When "that guy" buys it, you have this intuitive understanding that it's time to let the product go. It might also be the last purchaser within the threshold as well. Perhaps you had them in mind when you initially purchase the product. Now that "that guy" has it, you've saturated your market.
"That guy" might be a particular fringe buyer. For example, all stores have customers who have great taste, but are bell weathers for failed games. I have a miniature game customer who I joke with about this. He'll walk in and I'll ask him what has been interesting him lately, joking that we'll have to discontinue that. It's all in good fun, but he acknowledges that every game he plays tends to fail. He's all about what's cool.
Board games have a very rough sales curve, with most games only good for, at best, a single purchase, while a hot game can easily become "evergreen," selling for years. Often it's trial and error to find those games, although doing a lot of research before you buy can eliminate the dross.
I should say that I'm probably "that guy" too, which is why I have to be especially careful when purchasing. I used to have a larger income, so cost-benefit took a back seat to "cool stuff I want." The Ziterdes German terrain we brought in is probably a good example. Super cool, too expensive. I want. Store owners like me can do really well if we can channel what we think is cool through the filter of "what my customers will buy." It's still more art than science.