Best tips for personal safety
Always have at least two adults present during your garage sale
Three would be better as there will be times during the day when one person will need to quickly drive around and check the signs are still up or prepare lunch, etc.
Position yourself at the front of your garage in view of the street
Apart from helping to make sure your garage sale doesn't look deserted (a bad sign to people as they slowly drive by) it is safer for you than being at the back of your garage sale where you can't be seen from the street.
Don't include your phone number on signs or give out your phone number
If someone wants to check if an item is still for sale near the end of your garage sale, get their number and ring them. A thief with your number could ring later to check if you are home or not.
If someone is insistent that you short-changed them and are getting aggressive then it isn't worth getting into a physical confrontation over $10 or whatever the amount. Give them the money and send them on their way. Make a note of their description though in case they come back to try it again with another purchase.
If someone is confrontational for some reason, check that it isn't just a tactic to distract you while an accomplice 'pockets' some high-value items. A good way to counter a confrontational person is simply suggest that you will have to contact the Police.
Don't worry about 'shop-stealers'
You won't be able to realistically apprehend someone who you think is shop-stealing. You would be putting yourself at risk physically for little gain. It's better to make yourself visible to your customers by walking around tidying up and re-arranging items (and you'll also be making yourself easily available to make a sale). Ask what they are interested in and whether you can help. The close scrutiny will dissuade them from trying anything.
If something is stolen, it depends on how much it's worth to decide if you should bother filing an insurance claim.
Don't argue with a thief with a weapon!
Nothing is worth your life. If they want you fanny pack, cash box or whatever, immediately throw it gently to them ensuring you keep your distance from them. Hopefully you've been taking money inside when it builds up so there shouldn't be too much to lose. That should keep them happy but if they say there should be more then just tell them that that is all you've sold. It's a garage sale for crying out loud.
Keep a mobile or cell phone on you at all times
Just in case you need to call the police. And make sure it's charged the night before.
Finally, if you are still concerned about safety, don't have it at your own home
Rent space or a table at a community sale or flea market instead. You won't be on your own and no-one will know where you live.