I’m always wary of buying anything consumable (including things like toothpaste) for humans or animals at a Dollar Store because they usually (even brand names) are manufactured in countries that may not have as strict quality requirements as the U.S. Also be aware of the expiration date on products. While the dates are almost always far on the side of caution and I believe in the U.S. it’s not legal to sell if the date is past, be sure to check before you buy.
The store could also be buying overstock or liquidation items from other stores, liquidators. In Hawaii we have (or had, not sure if it’s still there) a discount grocery store that has these types of products.
I had a friend from Taiwan (whose father owned a cell phone accessories factory) that they made different quality products, sold at different price levels and on the lowest quality/price phone chargers they would omit a part that would prevent overcharging. In addition, the biggest cost of manufacturing products with a plastic shell is making the injection molds. These molds have a finite life when they’ll produce quality fitted parts, but are then often sold at their end of life to other factories who continue to use them despite the parts not fitting together properly.
Finally, the store may be a front for other illegal activities including money laundering. Since it’s a bargain Dollar Store, paying in small bills is not unusual. The store “re-buys” the goods less a percentage cut and restocks their shelves.