My experiences sound almost identical to yours. The curse of the garage sale - sometimes rude, in-your-face dealers rummage around your stuff at 7.00am as you set up for an advertised 8.00am or even 9.00am start.
One couple I talked to in Brisbane said that some dealers had turned up at 4.30am to their garage sale!!!
Everyone tells you to expect it - as it's the truth.
I did lots and lots of car boot sales - the UK equivalent - before we emigrated to Oz in 1996, and some guys would be looking into boxes in the boot of my car as I was unpacking. Still you're there to make a little cash, so you smile and accomodate them I'm afraid. The customer is always right!
I found that a lot of people expect you to give half decent goods away at ridiculously low prices though. A small cabinet that I couldn't get rid off to one guy for $5 went for $120 when I posted it on ebay!!!
They are lots of fun though - and help to fund the gear monster in all of us.
Cheers
Doug