For niche market, doesn't it cost a lot to carry a variety of products?

That's why Ellen recommend you to carry only 30 units for each product. If you carry 20 different products, if 2 of them don't sell, you still have 18 others.

If you carry popular products (or you thought it's popular products), and you carry 500 units each for 2 different products. If 2 of them don't sell, you have nothing left but a bunch of products that are not moving.