How do you know what to price somthing at garage sales?

Have a "free" box for all those plastic cups/containers, odds & ends you want gone.

Books don't bring much money - have a box Labelled $ .25 and fill it with books.

Clothes - unless it's an expensive suit or gown, clothes don't bring much money. Charge anywhere from $ .25 for socks to $3.00 for a dress. Nice coats perhaps $5.00. Nothing higher.

Electronic equipment is in demand. If selling anything electronic (toaster, lamp, etc) have an extension cord available for people see if it works. $5.00 up to ??? depending on the original price and what it is.

Group items rather than pricing individually, except for the big items (dining set, sofa, etc).

Again, if you want it gone, price to sell and be quick to negotiate. If there's a particular price you want, price it higher to give room for "haggling". People like to think they got a good deal just by asking.