some things at the dollar store may be found for less than a dollar elsewhere, for example Walmart. one thing i can think of off hand is abrasive cleanser. i remember seeing in in dollar store for a dollar, i know at that time (not sure about now) it was like 79 or 89 cents at Walmart. so you have to be careful, just cause it costs a dollar and that may seem cheap does not make it such a good bargain.
also, they probably buy “odd lots” for pennies on the dollar, then they sell it to you for a dollar. that’s why the merchandise there is constantly changing.