A strange nonlinear problem or question has shown up again.

I was doing the books and came across something interesting.
I work in landscaping and have noticed that of the 80 different products we sell, from landscaping stones to gravel. My sales of each item seem to be increasing and decreasing somewhat at random. Though I cannot find any seasonal, cost, or any other reason for that matter, looking at sales over 1 or more years shows that the same amount of items are being sold. At first, I thought that I must have made some simple mistakes or something. The sales information keeps coming out the same, 80 products selling in different amounts, at different times, and to different customers for no reason that I can find producing the same amount of sales over a longer period of time.
My question is, does anyone know of any way to determine which products will be in greater demand and which products will not. This would be of great help as I would not need to keep so much inventory on hand. I do my own bookkeeping, currently use Excel 2007, and have been observing this oddity for some time now.
Anyone got any ideas?
Basically, I just wanted to know if excel has the ability to project sales based on past performances and work backward keeping those sales within norms to show what is most probable.
For example, if one through eighty items looked at over time typically seem to maintain an average of 0.25 or there about. Can excel project all sales for a year or two and work backward to show which of the 80 items are most likely to be sold over the short term?
If this is possible, I do not know what function would do this if any. As I said before, I would like to be able to determine which items have a greater probability of being needed to maintain in inventory those products that are most likely to sell. This would reduce my operating cost and my profits much better.
Thanks in advance

BeachRock,

Thanks for the idea; I am currently investigating the oddity in a year-by-year fashion, suggested to me by another member of the forum. Call me nuts or something, I just cannot give up on curiosity. I am probably going to find that I am just being dumb or something, that is OK too.
In the meanwhile, “Time to make the doughnuts”
Thanks again,