yes you can lose if it drops overnight.. but.. if you buy at say 0.027, sell at 0.028 you have made a profit. you come back next day and price is 0.026, you sell and then you buy at 0.025.. your still making profit. the overall downtrend is irrelivant when you operate like that as you are always making a profit over what you have sold for regardless of the fact it is going downwards in the long term.
That's not the issue, you can (and have to) always rework losses after they occured.
The problem is the following :
If you come back next day and price is 0.025, you sell and you buy at 0.024... you
have realized a loss. Buying high and selling low is not called profit
According to your logic, anyway you'll buy at 0.024 to continue trading.... and price drops to 0.022 (assuming you don't stare at the screen every minute)... you have to realize losses again and repeat. Likely, odds are the next trade will be profitable, but you (usually) need 2 good trades to make up for one small loss that are in line with everyday expectations.
If you wake up to find price not at 0.025, but at 0.018.... You need
alot of good trades to make up for that surprise. Plus, your total available trade volume has dropped, making things more difficult.
That's the whole thing.
If it was so easy to buy low and sell high - every dumb tradebot would do it and everyone trading @ cex would be rich. Doesn't work that way, however. You still need luck.
I've been lucky in 19 out of 20 trades using a similar strategy (except I never held GHs overnight) during my experiments there but I was always fully aware that luck could change any second.
Turns out a week after I left in December, cex.io saw its first, massive overnight price crashes. Lucky me, that was all that prevented my (tiny) investment to get almost wiped out.