It may seem buggy but actually it's related with the budget of your customers.
Now that, you unlock new customer types by placing items with luxury points on your store, you're also getting able to sell the related products. For example, if you don't have enough Luxury Points to unlock the Hobbyist Customer (needed to sell the Treadmill), you will see a warning on this product.
But, you will also realize that you can order and serve this product and it's normal if you're thinking why you can order a product that you can't sell.
It's because there is a specific way of selling these products. Since, Shopper Cards increase customer budgets, if you give Shopper Cards to Geek customers, it will increase their budget and they will be able to get products from that stand (Speaking for the Treadmill example),
The developers are looking into ways to make this more clear. Thanks for your understanding!