1) They ENJOY having garage/yard sales. I suppose there are a scant few out there that fit this one and as they say down south, "Bless their hearts" - (personally I think they need their heads examined) - but to each their own and I commend them for bravely going where most of people won't even go.
2) They just want to be rid of their excess stuff and either don't want to or can't haul it to a donation center [or the Misses plans to get rid of those old golf clubs that haven't been used in 10 yrs. (among other things) and can't alert her husband by asking to borrow the truck]. Those in this group may also be part of a larger neighborhood sale, making it a perfect opportunity to get rid of things without all the hassles of doing a sale solo.
3) They need money! I fall under this one and #2. I have to get rid of almost everything I own before I move into my mobile 50 sq. ft. tearbaby and I need to make all the money I can before then.
Now, people who come to garage/yard sales are looking for a deal - I understand that - but I have a real problem when they try to take advantage of you. C'mon people! If you don't like the price, then you are free to go to Wally World and pay a whole lot more! If I wanted to give it away, I have a very long list of people I'd give it to long before I'd give it to these folks! I'd rather donate it then sell it to some people.
If I may share a personal request - if you are a garage/yard sale shopper, please keep in mind that some of us are struggling and need to make all the money we can however we can. Taking advantage of the person selling their stuff isn't cool. If you think it is overpriced and you really want it, make a reasonable offer and see. If they say no or counter back with something in between, then don't keep badgering them into going lower. Gracefully decline or say "WOW! Thanks, I'll take it!". Remember, you are still saving money from buying it new, reusing something and potentially keeping it out of the landfill, and are paying it forward by helping someone who might really need it.
Now go Shop Considerately! (And stay strong - keep walking past that gently used Elvis Soap on a Rope - even if it is only 25 cents)