Early Birds at Garage/yard Sales?

I want to have a yard sale this weekend and have been going to a few and doing my research. I see a LOT of ads for sales that say "No Early Birds!!!" yet when i spoke to one gal holding one , she said she thought that was the dumbest thing because you'd lose sales.
So exactly what do early birds do that piss people off so much besides showing up early?
I definitely went with the grain and wrote that as well on my ads, but I'm just curious what the worst is that happens.
Thanks
 
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