Watching for market trends takes time and effort. One is what the vendors have to offer that is new. The other is looking for something that hasn't been seen in past shows and is just emerging at this show. Learning and seeing trends requires lots of reading and attending shows for more than two years. After the first show, buyers are able to distinguish what is new and what was shown at the last show.
Every item cycles differently.
That is true with items that are related by category, size, price, color, etc. I like hand counting because it keeps the mind aware of what is and is not selling. These systems can be as simple as a tally count. So far, I have not found an computer inventory control program that gives rate of sale or merchandise cycle. I believe in counting inventory on a rotating basis where everything is counted a least once a month or 6 weeks and as new similar items are received the records are corrected.