Keep the money ON YOU. I say it again, keep the money on you. You absolutely need someone to help you. You may have someone ask to use the rest room, if you allow them, someone needs to go with them and wait at the bathroom door.
Around here you get about 10% of what you orginally paid for the item. I usually mark it a little higher, so that I can come down a little.
If you have some proof that it is for MD, show the proof. Some people falsely say garage sales are for a charity, and they are not. You will get more money if they actually know it is for a charity.
I like to do garage sales, and we usually make a good deal of money. Have fun.