DEFO listen to Stoner on this one, he obviously knows what he's talking about.
I'll not go on as I could bore you senseless about statistics, and how useful/useless they are etc. But the key is to keep things accurate. If you use an average of the percentages you make per item, then it's about as useful as a chocolate teapot. Stoner's weighted average will be far more useful, but would still not necessarily be totally accurate.
THE ONLY way to do this, is to sum the cost of everything you sold in a period at the cost you sold it at, and in another column the cost you bought it at. The work out the one versus the other as a percentage of markup, as you have done for each individual item so far.
It's hard to describe without showing people first hand what the effects of dud statistics can be, and just how often people lead themselves astray! But then I have the inside knowledge, and in fact have conducted training sessions ironically titled "how to lie with statistics" in a past job.