And what have I learned over the past three years? This: When it comes to set prices, there is no rhyme or reason to the numbers. I don't think there is a single set that has shown any consistency in price, either rising or falling. Each month seem to be unique unto itself.
To help visualize this, I've made a line graph of the monthly prices for the six oldest sets I track. These six are also the most expensive, because they are the only sets I track that were released in multiple series.
Take a look....
....The lines look like a roller coaster at Great America. They're up and down. And often, significantly up and down. I thought the prices might stabilize starting with February of 2012. That was the first month of Ebay's expanded listing of completed sales. Prior to that, they kept only a couple weeks available, which meant that there were several times when I didn't get five completed sales. You can even seen on the graph that there were a few instances when a set had no numbers to average. Fewer numbers to average may skew things.
Starting with February, 2012, I've got five sales for every set. And yet, the lines for May - February are just as jagged as any other time.
I can learn a few things from the prices though. First, the 1969 set is typically the most expensive. It's not just that its the oldest, but I would put the reason with one card, #500, Mickey Mantle's last card. You can figure on $150 -$200 on that one alone. Luckily for me, I had that card from back in the day; otherwise my complete sets would start with 1970!
The graph also shows that the 1970 set isn't very popular. It was my favorite as a kid, but it seems that the collectors show no love for the ugly grays. It's the psychedelic '72 set that gets the collector's love. It usually comes in with the second highest price. You can blame that on the expanded checklist (787 cards) and a fairly scare 6th series.
Finally, I've learned that if you want to buy or sell a set, you have to be informed and patient. Do your homework and don't rush. You will eventually find the set you want, in the condition you want, at the price you want. Just keep your hands inside the car at all times and wait for the roller coaster to come down.