Even if they accurately charged shelf prices, these places are still a ripoff targeting the vulnerable. The list price is low but the per-unit price is astronomical compared to grocery store prices.
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I've been able to find good deals on some things at Dollar Tree. Usually the good deals were a smaller quantity of a normal-quality brand-name item. I mostly avoid the substandard quality items. But even sometimes substandard is OK if, say, you want to make your political demonstration sign on white foamcore (much cheaper than the art supply store, and you don't care if it's smaller, thinner, or outgassing) rather than on an Amazon shipping box.
There was a Family Dollar across from a large public housing project here, where I also went looking for deals, but the shelf prices looked like a convenience store. I didn't find out whether they were fraudulently charging even more at the register like this article describes. (I hope it closed because the residents knew there was an affordable Market Basket a 20-30 minute walk away, over the city line and train tracks, and they were able to get there and find the time for it.)
Sometimes the sign-makers are artistically inclined, and may have access to better materials.
The most memorable example was at the political demonstrations (and counter-demonstrations) leading up to the Massachusetts constitutional convention that legalized gay marriage. For the State House one I photographed (learning photojournalism on the side), the anti-gay-marriage people were mostly bused in, including a pair of angry-looking old nuns in black full habit, and handed out the same ugly stock sign. (There's an obvious joke that they couldn't find a graphic designer who was sympathetic to the anti-gay cause.) Separated from them, across a street was a huge counter-protest, with an ocean of all sorts of creative, colorful, and positive handmaid signs, held by generally good-natured and thoughtful looking people.