I found mine on eBay, NIB never used and purchased like 60 test sticks from someone else on eBay as well. No problems... allot of people sell their used CBE monitors on eBay because really no urine comes in contact with it. Once you've dipped your test stick in a cup of urine you replace the cap to the test stick before you instert it into the monitor.
The reason I bought the NIB one was because I lost out on all the other auctions I was bidding on due to work schedule or just not getting to the nitty gritty bidding in the last minutes. So I would have purchased a used one.
You use 1 test stick during the testing period and usually only 10 sticks per cycle, so if you were to buy a box of 30 it would last 3 months/3 cycles. Which you can also find those cheap on eBay as well.
I would take out the batteries before starting the monitor which I believe erases any "memory" it has. The monitor has a memory of sorts and according to when your fertile days are from the previous month determines when it has you test in subsequent ones. If a cycle is very long it may have you test more than just 10 days. It may also have you test less than 10 days depending on when you get your *peak* reading then back to low which I've had happen as well.
I really like my monitor and opk's are ok as well... last cycle and this cycle I'm doing both as well as temp/charting.