If you want to see a set that has really stalled take a look at Diagon Alley. Back in 2012 it was thought of as one of the best sets you could pick up for investing. In the last 6 months it has seen only 3.8% growth and only 62% growth over retail.
As for lower sales part of that has to do with the time of year. March, April and May are always slow. But also the number of resellers is really starting to show. Just a year or two ago a set on Amazon would have maybe a dozen or couple dozen 3rd party listings. Now many sets have hundreds of other resellers.
I've pointed this out before but no one really seems too concerned or responds to it much. If you look at the BP home page in the footer you will see some site stats. I have been keeping track of these for a while now. Here is one stat that really has me thinking there are too many investors buying very large amounts.
January 31, 2014
Total current value of member Brickfolios: $ 8,537,670.56
February 8, 2014
Total current value of member Brickfolios: $ 10,497,434.00
February 26, 2014
Total current value of member Brickfolios: $ 14,688,530.08
March 19, 2014
Total current value of member Brickfolios: $ 16,999,452.07
March 25, 2014
Total current value of member Brickfolios: $ 17,317,887.53
As you can see in less than 2 months we have added well over $8 million dollars in inventory to our brickfolios. Of course some of this increase is due to our brickfolios increasing in value but that is only calculated once a month. When you see millions of dollars added every couple of weeks it gets a little scary.