Have a counterfeit money pen handy. I have had a few bad experiences with people passing off "bad" money. It's cheap and it could prevent a headache.
Where do you get one of these? We are having a sale this weekend and always get some big bills like 50's and 100's and are always worried about this.
I'm hoping to clear at least enough to pay for my new camera 
with 200 other people, what are the chances they will make it to your sale? However, if you start at 6:00am with maybe 5 or 10 other people, you will make huge amounts of money before 8:00am. I also see the buyers who buy the larger items (like furniture) come in the wee early hours. Many of them have 2nd hand furniture stores and they know that the early bird gets the worm.