Using this type of diary is a great way to help you improve in what you do because you can look back over what you've done and slowly but surely lower the amounts of errors your make. The common merchanting error a lot of people make isn't in picking the wrong item, most people are quite good at picking the right item but they're bad at knowing when to sell. It's far better to make lots of small to moderate gains then to lose constantly and only get a few big wins. When you've made a tidy profit then get out and keep it, eventually whatever it is you've bought into will drop and the longer you hold onto it the greater the chance of it dropping in value. I'd definitely say as a rule that if you're making 5-15% you should definitely sell the item, if you wait for it to crash not only will you not make any money but there's a massive chance you'll actually lose it.
What's more important is that lots of small victories are far better than a few big ones. Constantly making the right decisions will clear your mind for the trades to come and it'll give you a much better mind set. What you definitely don't want is to be sitting at the edge of your seat completely nervous because you know you're probably going to lose money!