Once you buy them, will they always turn a profit at some point before they rot? I'm only asking because I've just bought them one time and the continually declined in price over 5 days straight so i just sold them at a loss.
it won't always turn into profit. its all luck and there are guides to spotting your pattern of increase/decrease on YouTube. you can also sell at other islands with high prices, there is also a forum for that here on TBT!